Add default selectRootForWindow function for target root id

When window has options to specify the target root id, by default it
should be attach to the target root.

Also cleanup some documentation.

Bug: 155441180
Test: atest WmTests:DisplayAreaPolicyBuilderTest
Change-Id: I9cbfc974cd502a6cadd919b169cb9b8f9ce634f7
2 files changed