Rename output switcher grouping flag

This flag was originally not listed as a bugfix. The suggestion from the
release team is to re-define the flag.

Test: atest
Bug: 394907674
Flag: com.android.media.flags.enable_output_switcher_device_grouping
Change-Id: Icb50a5a013c29b0a3267b8b1ca192f05f0ecfcc0