Adds assist geture hint handles to ScreenDecorations

On screen gesture hint for using the corner gesture to
trigger assistant. This provides a placeholder for the
view within ScreenDecorations, to relocate from the
original location in StatusBar.

SysUI AssistManager will control the visibility of the
handles, in response to events. By default the handles
remain hidden.

Bug: 132285072
Test: manual visual verification while developing
Change-Id: I0234ce4677c77c9f4367334c1df7f4a8b59e8d66