Fix WindowMagnificationControllerTest failure on foldable targets

In foldables, the bottom valure of system gestureinset is 0.
We fix it with a test windowinset.

Bug: 196931900
Test: atest WindowMagnificationControllerTest
      use forrest to run WindowMagnificationControllerTest
Change-Id: Iaa58de87a89bf47edd9ac9beb52afa5b3cf006e7