audio policy: add offload gapless support flag

Add flag AUDIO_OUTPUT_FLAG_GAPLESS_OFFLOAD to explicitely indicate support
for gapless transitions for offload output profiles in audio policy
configuration file.

Bug: 158191844
Test: atest AudioTrackOffloadTest
Change-Id: I711f5677075e8a0f33c951656c8a019255e2f41c