audio: Make I/O operations in default stub more realistic

1. Increase the minimum buffer size to 256 frames. This is
   more realistic than 16 frames: 256 frames is ~5ms @ 48 kHz.

2. Make transfer delay in the stub module dependent on the frame
   count for synchronous transfers.

Bug: 270154517
Test: atest VtsHalAudioCoreTargetTest
Test: atest android.media.audio.cts.LoudnessEnhancerTest (w/AIDL enabled)
Change-Id: If968e30d145b52220f4dc3c33af48dbc163c78cd