Partial fix for jumping freeform windows

Currently every focus change causes a freeform window to jump
for a few frames. The size of the jump is equal to the difference
in inset size (which is derived directly from the window elevation).

This problem goes away for most real use cases if the insets are
not allowed to decrease.

Bug: 28318973
Bug: 22668382
Change-Id: I77ca440a7d9c89cc4d45e6667bf37da94a5c8a9a