Fix ToggleScreenMagnificationPreferenceFragmentTest

**Root cause**
ShadowFragment makes the test using an empty implementation of onCreate,
hence the test execution throws SuperNotCalledException.

ShadowFragment was accidentally added to this test when migrate to the
latest Robolectric. Removing ShadowFragment in the config fixes the
test.

Fix: 300158627
Test: atest ToggleScreenMagnificationPreferenceFragmentTest
Change-Id: I8782ac11a93c5f89b781dcca1713968ed18802cf