audio: Clarify "mute" behavior for playback parameters

When the client specifies `TimestretchFallbackMode.MUTE`,
it should still be allowed for the HAL implementation
to return an error if parameters are out of range and
it does not support muted playback.

Bug: 381385909
Bug: 384626226
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I757856dadd9cf234daa3554b0ca33e7b4e2e4f00