Add rounded corners attribute to window animations

Windows now can have rounded corners that match the display shape,
this CL introduces a new xml attribute called hasRoundedCorners that
enables this feature when declared on an animation xml.

Bug: 111514493
Test: winscope
Test: atest WindowAnimationSpecTest
Change-Id: Ie7651bab00c6e9e0ab7ab50cdb14142c8b2b954b