Isolate changes for output switcher grouping behind flag

The changes in b/31912901 seem to be causing crashes. This change looks
like it should've been a no-op for the non-flagged changed, but moved it
behind a flag anyway to isolate the issue.

Fixes: 399104884
Test: local test, atest
Flag: com.android.media.flags.enable_output_switcher_device_grouping
Change-Id: Ibf4fbf9bd696b4aadc7633b2da602e3a544b7ba0