Rewrite AudioMix validation & add more test coverage

LOOPBACK mode flag implies:
* only remote submix devices can be specified
* if no device is specified the device type will be inferred
  from mix

RENDER mode flag implies
* device must be explicitly specified
* only output device can be specified
* only MIX_ROLE_PLAYERS mix can be configured in render mode

For LOOPBACK & RENDER mixes, both sets of requirements must
be fullfilled.

Bug: 287908341
Test: atest AudioHostTest AudioServiceHostTest AudioMixUnitTests
Change-Id: I43184f3ac2ecd748da5ea8f0e5dc1be70270e254