Fix flaky CarAudioFocusTest

Modify the scope of locks in requestAudioFocus in AudioManager so
that method dispatchFocusResut FromExtPolicy is called only
after clientId has been already added into
mFocusRequestsAwaitingResult. Meanwhile, increase
EXT_FOCUS_POLICY_TIMEOUT_MS time since the method notifyResult
sometimes reaches 200ms timeout when the emulator device is slow.

Bug: 234467798
Test: m -j
Test: atest CarAudioFocusTest AudioFocusTest
Test: atest com.android.car.audio
Change-Id: I0198549bbae0b4e78523e244962a71a24be4dde8