AudioMix: add option for silence injection on injection starvation

New option in AudioMix's Builder to inject silence for audio
injection mixes when the injector is starving the mix (not
providing enough audio data in time).
Also annotate other Builder methods with @NonNull when returning
the Builder itself.

Bug: 416965224
Test: atest CtsMediaAudioTestCases:AudioMixTest
Flag: android.media.audio.dap_injection_starve_management
Change-Id: I7838702e5a21346954c5f1a18dbaa6b3ae0a8861