Better IME transition while switching app with recents (6/N)

Add WindowState#{freezeInsetsState, clearFrozenInsetsState} to freeze
the insets state of the window to keep the insets state when the window
is in app exiting transition, to ensure the exiting window won't
receive unexpected insets changes from the next window.

And, in order to easy maintain the logic of dispatching all windows's
insets changed event in InsetsStateController, instead of adding
complicated rule in InsetsStateController to judge if the window can
disatch insets change, we can replace with
WindowState#isReadyToDispatchInsetsState() to wrap the dispatch
judgement.

Bug: 166736352
Test: atest CtsWindowManagerDeviceTestCases WmTests
Test: atest WindowStateTests#testSetFreezeInsetsState
Test: atest WindowContainerTests#testFreezeInsetsStateWhenAppTransition

Change-Id: I56418733c1abbd73e88a8918a5d55ecc15344c5e