[Magnifier-57] Add API to set overlay

The CL adds an API to customize what overlay will be drawn on the top of
the magnifier content. Our default is to draw a 5% white overlay to make
magnifiers distinguishable in dark contexts.

Bug: 72211470
Test: manual testing
Change-Id: I1a356813960a60f49e068c6135ded9d41429d57c