FocusRequester: refactor handleFocusLoss

Break out the checks to see if a focus loss is handled by the
framework in a separate method, and reorder the if statements
for code clarity.

Annotate PlayerFocusEnforcer methods for nullability, add
javadoc

Test: atest AudioFocusTest
Change-Id: I9aaab804c78a2e2627ddbd7b9bd41fdec47d2b57