Canoncial IME Target (2/N)

Introduce DisplayContent#getImeTarget to generic
getting IME target method in accordance with IME_TARGET_.* flag:
 - IME_TARGET_LAYERING => mInputMethodLayeringTarget
   (a.k.a mInputMethodTarget)
 - IME_TARGET_INPUT => mInputMethodInputTarget
 - IME_TARGET_CONTROL => mInputMethodControlTarget

Added setter for setting IME related targets, also makes the
IME target fields as private for ecapsulation.

Bug: 163453496
Test: atest WmTests
Test: atest DisplayContentTests

Change-Id: I3ad8bfec218de8708e372704ce9852053f7c8542