Add US-MTV and TW TLs to media solutions OWNERS

This ensures that someone can approve emergency rollbacks and fixes
outside of LON hours for files exclusively owned by media
solutions, as configured in
https://android-review.git.corp.google.com/q/topic:%22media-playback-update-OWNERS%22.

Bug: 285890867
Test: None (OWNERS-only change)

Change-Id: I3d0877a1928d3dfc359e420751ada1a6ae7fb106