Fix @UiThread crash when audio devices change on Android TV (fixes #188)

[under3415]

PR Description — Fix @UiThread crash on Android TV HDMI devices

Title:

Fix @UiThread crash when audio devices change on Android TV (fixes #188)

Body (paste into the PR description):

Fixes #188

## Problem

On some Android TV boxes, `AudioManager$AudioMonitorHdmiThread` invokes
`AudioDeviceCallback.onAudioDevicesAdded/onAudioDevicesRemoved` on a background
thread, ignoring the main-looper `Handler` passed to
`AudioManager.registerAudioDeviceCallback()`. The callback then calls
`MethodChannel.invokeMethod()` (annotated `@UiThread`) from that thread, causing
a fatal crash:

    java.lang.RuntimeException: Methods marked with @UiThread must be executed on the main thread.

Affected devices include Skyworth TV boxes (Android 10), Inspur set-top boxes
(Android 11), and low-end Fire TV devices. Phones are unaffected because the
HDMI audio monitor thread does not exist there.

First report: https://github.com/ryanheise/just_audio/issues/1484

## Fix

Guard `AudioManagerSingleton.invokeMethod()` so all MethodChannel invocations
are dispatched on the main looper. This is a single-point fix covering all
callback-originated events (audio devices added/removed, focus change, becoming
noisy, SCO audio state), rather than patching each callback.

If already on the main thread, dispatch directly to avoid an unnecessary hop.

## Verification

- `flutter analyze` passes
- `flutter test` passes (example app)
- Runtime-tested logic: on-main-thread path identical to previous behavior;
  off-main-thread path is now marshalled to the main thread