Match default magnifier window size to Medium in settings

We would like to initialize the magnifier to be Medium size, which is
one of the sizes that provided through magnification settings panel icon buttons, so that users could get back to the initial window size easily.

Since we are following the sizes provided by the settings panel, we will
need to rethink if those provided sizes are appropriate and useful for
users with different screen sizes. We would like to deal with the issue in a separate bug.

Bug: 265363901
Test: manually - check the magnifier is initialized to be Medium size
Test: atest WindowMagnificationControllerTest
Change-Id: I37762e002cb766b06f99817c87a25c9539faed6d