Let IME target window per display.

Since currently in WMS only recorded one IME target window that
the IME window on top of.

For multi-sessions IME on multi-displays scenerio, we need
move WMS.mInputMethodTarget to DisplayContent.

Added unit test testInputMethodTargetUpdateWhenSwitchingOnDisplays
for verifying per IME target window update when switching on displays.

Bug: 117962777

Test: atest ActivityManagerMultiDisplayTests
Test: atest FrameworksServicesTests:ZOrderingTests
Test: atest FrameworksServicesTests:WindowContainerTraversalTests
Test: atest FrameworksServicesTests:DisplayContentTests
Change-Id: I9aa934961fb3975bd2af18599b5a2884387b5007