Use semaphore instead of condition variable

BackgroundExecutor uses a condition variable to synchronize processing
to a work queue, which means that there is possible mutex contention on
the main thread in rare scenarios.

Instead, we can make the main thread non-blocking with a thread-safe
queue, and use semaphores for signaling the background thread to wake
up. Using semaphores saves a small number of instructions, but overall
reduces contention

Bug: 232113929
Test: bouncy ball

Change-Id: Iced25e8349bdb2a70cac1ed681059a0b14258407