Fixed issues with child windows been IME targets

- Fixed issue with WindowState.getWindow() returning the parent window before
its children with positive sub-layers. Positive sub-layer children should be
returned first, then the parent window, and then negative sub-layer children.
This was causing the the parent window to be selected as the IME target
instead of the child on-top of it since they both can be IME targets and the
parent window was returned first.
- Fixed issue with WindowState.forAllWindow() not returning the IME window if
the current IME target is a child window.
- Add test WindowStateTest.testGetWindow(),
DisplayContentTests.testForAllWindows_WithChildWindowImeTarget(), and
DisplayContentTests.testComputeImeTarget() to cover the failing cases.

Change-Id: I0c93e0344601fc870011e8a8d84528f62b0a2a06
Fixes: 34786357
Fixes: 34306127
Fixes: 34711958
Fixes: 35362942
Test: bit FrameworksServicesTests:com.android.server.wm.WindowStateTests
Test: bit FrameworksServicesTests:com.android.server.wm.DisplayContentTests