Keeping the magnifier window size in SharedPreference

Using the smallest width dp as the key to save magnifier window's width and height. If no corresponding saved window size is found, use the Medium size provided by the settings panel button to be the default magnifier size.

Bug: 265363901
Bug: 283019152
Test: atest WindowMagnificationControllerTest
Test: atest WindowMagnificationSizePrefsTest
Test: manually - test window size is restored after the device restarts
Change-Id: Id35c2ff394c9a4907240833f3d43b0d412ef6caf