Store and return ImeContainer in RootDisplayArea for layers

Before, calling RootDisplayArea.findAreaForToken for TYPE_INPUT_METHOD
returns null. Now, it should return the ImeContainer if it is attached
below that RootDisplayArea.

Fix: 173478743
Test: atest WmTests:DisplayAreaPolicyBuilderTest
Change-Id: I28cc3223a0b9870accff56f93f4cb0fb87b0923f