Flush binders in Async LiveData

Ensure binders are released by liveDatas that use coroutines. Also
changes thread pool size from 16 to 8, in order to help avoid binder
exhaustion.

Fixes: 153603849
Fixes: 158207248
Test: I wish. Everything still works, and "adb shell cat
/d/binder/proc/$pid" shows fewer completed but not released binder
allocations.

Change-Id: If4561fd304adace419e27e871980322b3ccb8c04