Add flags to stash taskbar for app pinning and empty hotseat

Test: pin an app from overview, watch taskbar stash automatically until unpinning
Test: turn off suggestions for hotseat, remove all items and watch taskbar stash automatically when opening an app
Fixes: 190192993
Fixes: 193937948
Change-Id: Ia7260c60a820af1a48c9e4a400a52753baf34d41