Don't allow focus on activity in minimized split-screen.

Return false for ActivityRecord.isFocusable() for activities in minimized
split-screen since their windows can't be focused or interacted with.
Also, move focus off split-screen primary stack when we enter minimize
mode.

Change-Id: Iecec5d55123f1c3530c461314fa69498575ef5c3
Fixes: 74404706
Fixes: 74837107
Fixes: 74919175
Test: Put an app in split-screen, launch it from launch again, and observe ANR.
Test: atest ActivityManagerAppConfigurationTests#testDialogWhenLargeSplitSmall
Test: atest ActivityManagerSplitScreenTests#testNoUserLeaveHintOnMultiWindowModeChanged