AudioAttributes: set content type to UNKNOWN for init from stream

When creating AudioAttributes from a legacy stream type, do
not read the content type from the audio strategies, or set
it to a specific value for STREAM_MUSIC.
Instead use CONTENT_TYPE_UNKNOWN which more correctly
represents the fact that stream types are not enough information
to express the content type for STREAM_MUSIC.

Bug: 301100160
Test: atest android.media.audio.cts.AudioAttributesTest
Test: atest android.media.audio.cts.AudioManagerTest
Test: atest android.media.audio.cts.VolumeInfoTest
Test: atest android.media.audio.cts.NonDefaultDeviceForStrategyTest
Change-Id: I7299465a690aeeac953edf61aab0fa9d350c8f65