Add a flag for the Output Switcher UI updates.

Add the flag to the `media_better_together` namespace instead of
`systemui`.

Flag: com.android.media.flags.enable_output_switcher_redesign
Bug: 388296370
Fix: 388296370
Test: Exempt, adding a flag.
Change-Id: Iedc6795cd20d6274788fd2649fd982b3196e594e