BT: add LOW_LATENCY mode if low latency allowed

LatencyMode is not updatable after audio session started.
Currently, if low latency is allowed, audio session should have
LOW_LATENCY latency mode.
Create a workaround to add LOW_LATENCY when low_latency_allowed_ set to
ture at ReportLowLatencyModeAllowedChanged.

Bug: 291760415
Bug: 294498919
Test: play low latency audio with supported headset and check
LOW_LATENCY mode can be selected

Change-Id: Ia324f0a368e47ae8b8056c190d8d1f4a2e09ecec