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
Test: play low latency audio with supported headset and check
LOW_LATENCY mode can be selected
(cherry picked from https://android-review.googlesource.com/q/commit:25b4819e08280ea1bffa019a231e1d60edea7338)
Merged-In: Ie0626179a56334125cd48a048d162f09882888eb
Merged-In: I9d5510116225aa5857d1bd086956bc46d46a3eba
Change-Id: Ia324f0a368e47ae8b8056c190d8d1f4a2e09ecec