Window Manager Flag Migration (2/n)

Adds policy to decide when focused window can control bars. And also
generalizes control target for:
a) Normal case: The focused window is the control target.
b) Transient bar case: The control target is a special object in WM
   that controls the transient animation.

Bug: 118118435
Test: atest InsetsPolicyTest InsetsStateControllerTest WindowStateTests
Change-Id: I4819d85eec745d56abe23dbca0905979da854e5e