PiP: Double-tap to maximum, again to go back to last resized size.

When double tapping, it now maximizes PiP to the biggest size possible,
but doesn't save the maximized bounds as a user resize. Instead, save
the current bounds before maximizing, and then restore that if user
double taps again.

Bug: 178881304
Test: Double-tap on PiP
Change-Id: Ib1dfb07e1ee1134759309213089504bb1eb97279