Clear all DragView in onSaveInstanceState

Multiple users reports view getting stuck on screen. This CL
clears all DragView when user leaves launcher.

Root cause still undetermined, but happens enough that it warrants
a temp fix.

Bug: 162636845
Test: manual
Change-Id: I532b5274a1308c53f67c23c5c8ca07f2b7f9802c