Callback custom PiP action as Button

For Accessibility, callback the custom PiP actions provided by app as
"Button", instead of asking each app to set so. This aligns with other
system provided actions, such as Expand, Settings and Close.

Flag: EXEMPT bugfix
Bug: 373840926
Test: Manually check TalkBack
Change-Id: Id577f88a436cf7579545843918600eaaf4e7007f