WM: Refactor DisplayContent in preparation for DisplayArea (1/n)

To allow for DisplayArea, the following needs to be hierarchized first:
- IME window traversal & layer assignment
- Splitscreen layer assignment
- Rounded corner layer creation and assignment.

While we're here, we'll also clean up the display re-positioning, which
will interfere with DisplayArea repositioning.

Bug: 147406652
Test: atest WmTests
Change-Id: I53b816ec0450070847d582ce12fc50b40d7fceef