[bitsandfoxes]

Follow-up to #2895. On Android with Unity as a Library, game hangs were never detected when the game attaches to the app's sentry-android (Unity in the app's main process).

sentry-native's app hang detection only watches a thread that sends heartbeats (sentry_app_hang_heartbeat), and it does nothing until one arrives. sentry-android only passes enableNdkAppHangTracking on to native init and never sends heartbeats itself; downstream SDKs are expected to. When attached, the game skipped its heartbeat, so detection was never armed. The app's JVM ANR detection watches the UI thread, not the game thread.

Now, when attached, the game sends the heartbeat if the app's SDK has isEnableNdk() and isEnableNdkAppHangTracking() on. The app's options decide, as for all native options when attached. Unity's EnableAppHangTracking/AppHangTimeout can't apply there, because the app's sentry_init already set up the watchdog. As before, the C# watchdog is removed only when the heartbeat runs.

Unload/reload false app hang (both Android UaaL layouts)

Testing this turned up a false app hang after the app unloads and reloads the game, also in the :Unity layout. On reload Unity restarts Time.frameCount and Time.realtimeSinceStartup. The heartbeat coroutine was suspended across the unload, so it only resumed once the new clock caught up with the old one: on the device, ~27 s later, about as long as the game had run. Meanwhile the beat on resume had already re-armed detection, so a hang was reported ~5 s after the reload. The heartbeat coroutine now restarts on resume.

Verified on a Pixel 6 (Android 16, local UaaL host on sentry-android 8.59.0 with setEnableNdkAppHangTracking(true))

	Main process (attached)	:Unity
10 s main-thread hang	1 AppHang	1 AppHang
4 show/back cycles	none	none
app unloads the game, reload	none	none
Application.Unload(), reload, then a hang	1 AppHang	1 AppHang

Before this change, a 10 s hang in the main-process layout produced no event.

#skip-changelog: #2895 isn't released yet.

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