fix(magnification): Fix the test of saving and restoring window size preference

1. Using FakeSharedPreference in the test to guarantee the functions
   applied to SharedPreferennce are committed properly when we test the
   results.
2. Cleaning up SharedPreference contents to ensure the values stored in
   previous tests won't affect the next tests.
3. Verifying the width and height of view with the magnification frame
   size and margins.

Bug: 331139221
Flag: N/A
Test: Testing following tests several iterations
    atest WindowMagnificationControllerTest
    atest WindowMagnificatinControllerWindowlessMagnifierTest
    atest WindowMagnificationSizePrefsTest
    atest MagnificationTest
Change-Id: I3b287d5a9bf20db083f89a67c8ce5e9b19eec333