Set IME insets invisible if the target is above IME

We would remove the insets source of IME before dispatching to the
target above IME previously. However, when the z-order of an invisible
IME is changed, the client may receive unnecessary insets-change
callcacks (InsetsState with or without IME source).

With this CL, as long as IME is invisible, no matter what z-order the
IME has, the client will get the same IME source.

Fix: 152722388
Test: atest ChildlessActivityTest InsetsStateControllerTest
Change-Id: I9f147976be47a9fced4493dba10113b8ab048f4e