Support to get the exact task which IME is attached to

To distinguish which side of the split that IME panel is attached to
after having a single-top root task for split screen, update
getImeTarget API to return the exact task that IME panel is attached to.

Bug: 207185041
Test: WMShellUnitTests
Test: enter split in portrait, toggle IME panel on the bottom split and
      verified split layout will be push up as expectedly.
Change-Id: Ib6ba74ecb1b844bb408475d5b6c28dd5816a8e29