audio: Make StreamDescriptor::Command a union

Previously StreamDescriptor::Command used to be a parcelable
holding a command code (enum) + a command argument which was
used for one command only. This got replaced with an equivalent
union, where each command w/o arguments is represented by a
'Void' variant, and the command with an argument with a variant
of a corresponding type.

This has the following benefits:

  - the union guarantees that the argument is only set for
    commands that actually use it, thus all related comments
    and validations can be removed;

  - the command dispatch can be turned into an exhaustive
    switch.

To be able to use an exhaustive switch, the 'exit' command
has been exposed at the interface, but marked as 'hal_reserved'.

Bug: 205884982
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I3f2c27a48e4d0b7cfce1171244b2eddc1637005f