[flutter tool] honor inherited DASH__SUPPRESS_ANALYTICS - #194750

…4750)

When a parent Dash tool sets `DASH__SUPPRESS_ANALYTICS=true` (for
example, during the root tool's first run), the Flutter CLI now stops
reporting analytics itself. Before this change, `getAnalytics()` only
checked `FLUTTER_SUPPRESS_ANALYTICS`. Flutter passed the flag on to the
tools it launches (flutter#194261), but still reported on its own.

`getAnalytics()` now returns `NoOpAnalytics` when the flag is set. Like
`FLUTTER_SUPPRESS_ANALYTICS`, this also means no crash reports and no
consent message for that invocation. Only the exact string `true` counts
as a request to suppress, which matches `package:unified_analytics`, the
`dart` CLI, and `ErrorHandlingProcessManager`.

A no-op instance can't change the telemetry setting. So when the flag is
set, `--enable-analytics`, `--disable-analytics`, and `flutter config
--[no-]analytics` now print a warning that the setting wasn't changed
and why. Before, they reported success without saving anything.

Test notes:
- Some `runner_test.dart` tests now override `Platform`. The test
process can inherit `DASH__SUPPRESS_ANALYTICS=true` from the `flutter
test` that launched it: on bots, or when telemetry is disabled locally.
- Any new test of the opt-out/opt-in commands needs to override
`Platform` the same way.

Follow-up to flutter#194261.

(Fixes flutter/dash-devexp-internal#98.)


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