audio: Implementation of audio I/O, part II

This patch implements audio I/O for the synchronous, non-MMAP
case.

Updated the StreamDescriptor structure to make it usable.
Clarified comments on the expectations for the client and
the HAL module.

Bug: 205884982
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I09651c6e80a397c80870622ac19234b4d4a38cbb