Do not allow pip window to start background single instance activity

Otherwise, the window will be recreated and shown in foreground

Bug: 170613407
Test: Fail to start a singleInstance activity in pip while the app is in background
Test: atest ActivityStarterTests#testBackgroundActivityStartsDisallowed_unsupportedStartsAborted
Change-Id: I21cb91ab544557a5c8a4a1515604fa8f5c7797bb