Add isRound to WindowInsets

isRound allows a view to determine whether the window it is contained
within obscures the corners of the window content. This allows views
aware of this property to adapt their layout accordingly.

Switch ViewRootImpl to use dispatchApplyInsets instead of
fitSystemWindows.

Change-Id: Ic3e3936b73815b2593cb9720af1a309fbd18406e