Flag session changes that happen after a volume key press

This change makes session updates that happen right after a volume key
press include shouldShowVolumeUi = true.

Bug: b/396394220
Test: atest CtsMediaRouterTestCases
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: Id7154794171e4dc3447c734a3bb7041b080d09e2