Android Embedder Migration to Embedder API - #24

Report the display cut-outs as view padding (needs 0009).

0009 sends the cut-out padding on flutter-sfos/display_cutouts, and apps had to add it to MediaQuery themselves (flutter_sailfish's SailfishCutouts.builder), because the embedder API had no view padding. With the engine's view padding patch (patches/<ver>/engine/0001-view-padding.patch), the padding goes in the window metrics instead, so MediaQuery.padding, SafeArea and app bars avoid the cut-out in every app.

The embedder's FlutterWindowMetricsEvent gains the engine's size constraint fields (sent as no constraints) and physical_padding, a FlutterEdgeInsets as proposed upstream for the Android embedder (flutter/rfc#24), in the engine's order; engines without the patch ignore it. Metrics are sent again when only the content orientation changes, since turning upside down moves the cut-out without resizing. When built with FLUTTER_SFOS_VIEW_PADDING (the runtime spec sets it if the engine's flutter_embedder.h has physical_padding), the cut-out message says "in_metrics": true, so SailfishCutouts.builder doesn't add the padding a second time.

The engine patch added four physical_view_padding_* fields to FlutterWindowMetricsEvent. Flutter's RFC for moving the Android embedder to the embedder API (flutter/rfc#24) proposes the same thing as FlutterEdgeInsets physical_padding, and its draft prototype (flutter/flutter#195336) adds it in the same place. The engine patch and embedder patch 0014 now use that layout, read and checked as the prototype does, so our patch can be dropped once Flutter ships it. The runtime spec looks for the new field.

Tested on Jolla Phone 2026, Sailfish 5.2.0.20, in a sandboxed copy of an app

…Android (flutter#196342)

... and also make `PlatformViewAndroid::NotifyChanged` non-blocking at
the same time.

On Android,
[`surfaceCreated`](https://github.com/flutter/flutter/blob/e6b33e67d8cf6b53e1798438580e16453e4cc78d/engine/src/flutter/shell/platform/android/platform_view_android.cc#L189-L200)
and
[`surfaceChanged`](https://github.com/flutter/flutter/blob/e6b33e67d8cf6b53e1798438580e16453e4cc78d/engine/src/flutter/shell/platform/android/platform_view_android.cc#L240-L247)
currently block the main thread until the raster thread has finished any
queued `SetupImpellerContext()` work, created the swapchain, and set up
the rasterizer. Because this runs inside the first view traversal, both
the `Activity`'s first native draw and Dart's first frame wait behind it
(~80–110 ms on mid/low-tier Android devices with Impeller Vulkan, e.g.
~70 ms faster non-debuggable cold-start time-to-first-frame on a Moto
G05 once unblocked).

Per Android's `SurfaceHolder.Callback` contract, only `surfaceDestroyed`
is required to block until the render thread stops accessing the
`Surface`; `surfaceCreated` and `surfaceChanged` can forward surface
work asynchronously while `fml::RefPtr<AndroidNativeWindow>`
(`ANativeWindow_fromSurface`) keeps the native window reference alive
until teardown (see flutter#196180 for
more details)

### Changes

**`shell/common`:**
- Adds `PlatformView::NotifyCreatedDeferred` and
`PlatformView::Delegate::OnPlatformViewCreatedDeferred`, used only by
Android in this PR. The platform passes a `SurfaceFactory` callback;
`Shell` posts surface creation + `Rasterizer::Setup` directly to the
raster task runner (preserving FIFO ordering with subsequent
resize/teardown tasks on the raster queue), posts
`CreateResourceContext` to the IO task runner once a valid `Surface` is
created, and schedules a UI frame without waiting.
- Every other platform keeps calling `PlatformView::NotifyCreated`,
which is unchanged.
- Because `Rasterizer::Setup` can now run concurrently with
platform-thread calls that disable or read the raster thread merger,
protects merger access with `raster_thread_merger_mutex_`, starts a
lazily created merger disabled if `DisableThreadMergerIfNeeded()` was
called while setup was in flight, and resolves a spawned shell's parent
merger lazily on the raster thread.

**Android (`shell/platform/android`):**
- Updates `PlatformViewAndroid::NotifyCreated` and
`PlatformViewAndroid::NotifyChanged` to post to the raster thread and
return immediately. An atomic generation counter (`surface_generation_`)
skips queued setup/resize tasks from a surface that has since been
destroyed or replaced.
- Keeps `NotifyDestroyed` (and `NotifySurfaceWindowChanged` for
`FlutterImageView` resize) synchronous, and preserves existing
attempt-and-validate failure handling when surface creation fails.

Perfetto cold-start traces on Moto G05 (Impeller Vulkan):
[before](https://ui.perfetto.dev/#!/?s=c08b978984e818175e489d5bed5eb68c2e587ab7)
vs.
[after](https://ui.perfetto.dev/#!/?s=9225a8152d4e94a21d4a3e70e2b4c0de7cf3c578).

Fixes flutter#196180, and is part of
flutter/rfc#24

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