Add unit test for dynamic policy.

Add unit test for dyanmic policy. Including:
    1. Use bp file instead of mk file
    2. Test (un)registering policy mixes
    3. Test dynamic policy playback rerouting
    4. Test dynamic policy mix record injection
The idea of 3. and 4. is from pa/1201282.

Test: atest audiopolicy_tests
Bug: 135621476
Change-Id: Ic95c6f9193c2448168897c5441554cff735f401d