fix(magnification): fix AccessibilityManagerServiceTest on Wear

This test only makes sense if the test device supports Window
Magnification. Skip it otherwise.

This seems safer than mocking the package manager to lie about the
device supporting Window Magnification, since we don't know what other
parts of the system may have a dependency on that feature.

Bug: 299541066
Bug: 281124729

Test: atest com.android.server.accessibility.AccessibilityManagerServiceTest
Change-Id: Ice7a101fffb6a15b83d42c9ab12d992ecd269fe8