Expose main thread's looper for runOnUiThread compatibility

[markkimsal]

Posting to the UI thread is only possible via the looper obtained from the main activity thread. android-activity creates the AndroidApp inside a worker thread and there is no way to access the main thread's looper. Even interacting with the UI will send events into the AndroidAppInner looper and execute code in the worker thread.

This PR simply records a pointer to the thread's looper before spawning the worker thread and makes it available in a similar fashion to the existing looper.

Related discussion:
#197

[rib]

Hi, sorry for taking a long time to look at this.

Thanks, the idea seems good to me.

I've rebased this and added corresponding support to the game-activity backend.

I've updated the implementation to expose ndk::looper::ForeignLooper instead of raw ALooper pointers to avoid any ambiguity with needing to retain/release ALooper references (and you can still use .ptr() if you want the underlying ALooper API.

The other change I made was to refer to this as the "java main" Looper and not just the "main looper" to avoid confusion with the thread/Looper that's associated with android_main.

Based on this work, I have also experimented with building a more ergonomic AndroidApp::run_on_java_main_thread API that can be used to run boxed closures on the java main / ui thread and I'm planning to open a follow PR for that soon.

[rib]

For reference here, this is the follow up PR that adds a run_on_java_main_thread API: #232