Add helper functions to WindowFrameTests to avoid using assertRect()

This CL introduces some more explicit assert functions to
WindowFrameTests to handle the difference of behavior between
phones and ARC in those functions.
This CL doesn't affect the behavior in master at all.

Bug: 110907155
Test: atest WindowFrameTests
Change-Id: Icc86f18f605fdca82b1f358cb0e9d8cbd5f25b3a