AudioSystem: add annotation on API used by CTS media

  Add @TestApi for the class so some methods and
constants can be visible to test API.
  Also @hide for all other members of the class
that are not @TestApi, as they are exposed to the
test API visibility.

Bug: 136121584
Test: run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioProductStrategyTest
run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioVolumeGroupTest
run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioVolumeGroupChangeHandlerTest
run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioManagerTest#testPermissionsForVolumePerAttributes
run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioManagerTest#testGetAndValidateProductStrategies
run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioManagerTest#testGetAndValidateVolumeGroups
run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioManagerTest#testSetGetVolumePerAttributesWithInvalidAttributes
run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioManagerTest#testSetGetVolumePerAttributes
run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioManagerTest#testVolumeGroupCallback
Signed-off-by: François Gaffie <francois.gaffie@renault.com>

Change-Id: Iff9abda2cf971d9c234dcf7df7c2b9a4ad5294c3
Merged-In: Iff9abda2cf971d9c234dcf7df7c2b9a4ad5294c3