SpatialAudio: Set low latency allowed signal at startSession

The logic is that to send FREE and LOW_LATENCY to audio at startSession,
if bt receives setLowLatencyModeAllowed(true) before start session. If
the parameter false, or bt doesn't receive the signal at all, then bt
sends FREE to audio.

Test: build
Bug: 214615268
Tag: #feature
Change-Id: Ie2a7b9b2b387aa7b6f6ff6b14c0ee8ab0a3fcaf7