audio: Improve test coverage

Add a vendor-specific parameter "aosp.forceTransientBurst"
which helps to cover the case of non-full async writes.
This parameter is specific to the "AOSP as vendor" implementation,
other vendors are not required to have it—that's why it's
not in AIDL.

Fix minor issues in VTS found after revisiting the code, and by
looking at logs during testing.

Bug: 262402957
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Ide961d91a8d9da24392561654f04eb8207b7b781