Translate 'FALLBACK_DEFAULT' playback parameter into 'FALLBACK_FAIL'

For 'android.media.PlaybackParams', the SDK says that the fallback
mode 'AUDIO_FALLBACK_MODE_DEFAULT' means "System will determine best
handling." That means, the system (framework) must translate this
value into some concrete one before passing it to the HAL. Since
the initialization for 'audio_playback_rate_t' uses 'FALLBACK_FAIL',
use it as the "best" value.

Flag: EXEMPT bugfix
Bug: 431714495
Test: atest CtsMediaAudioTestCases
Change-Id: Iaf2f1ea57f5901546ce68d9f5e4750e179821e1d