AudioPolicyManager:Fix CTS testGaplessMP3AudioTrackOffload fail

getDirectPlaybackSupport return AUDIO_DIRECT_OFFLOAD_GAPLESS_SUPPORTED,
but getOffloadSupport return AUDIO_OFFLOAD_NOT_SUPPORTED cause test
fail.
solution: not return offload support when offloadPossible is false.

Test: atest AudioTrackOffloadTest#testGaplessMP3AudioTrackOffload
AudioManagerTest#testAdjustUnmuteNotificationInVibrate
AudioManagerTest#testStreamTypeAliasChange
Bug: 304449830

Change-Id: Ia68e70b98469e5c35b74e20e062519a4eb91d592