[various] Enable avoid_dynamic_calls - #6834

Enables the avoid_dynamic_calls lint, to bring us into better alignment with flutter/flutter lints.

There are some miscellaneous changes, but the large majority of these are cases where platform channel code was using dynamic values directly (usually for method call arguments). Common changes:

• Converting dynamic to a Map for method call arguments.
• Marking force-unwraps that were not previously flagged because someDynamicVarible as NonNullableType compiles, while someObject?Variable as NonNullableType doesn't and needs an explicit force-unwrap (due to another lint we already have enabled).

Since these changes aren't mechanical, and can cause runtime exceptions, they should be reviewed carefully (as we may well not have complete test coverage of all touched lines). In particular, please check that any as List<...> or as Map<..., ...> isn't using any types other than dynamic or Object?, as anything else is likely to be an exception-causing bug. (The correct construction for better typing is something like (... as Map<Object?, Object?>).cast<AType, AnotherType>().)

In general I preferred using Object? for updated code, per modern Dart style, but in some cases where there was already significant explicit use of dynamic I kept that style to minimize disruption. We may want to separately clean up areas of code still using dynamic at another time. Although ideally, we would replace most of the code here with Pigeon and avoid the need for manual, error-prone casting at all.

Part of flutter/flutter#76229

Pre-launch Checklist

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