Safely restore the property for legacy splash test

The original test read all of DeviceConfig, which was excessive
and on recent releases of Android can lead to a crash as services
can write properties that are readable by the test process but
are no longer writeable.

Fix: 427476703
Test: atest 'ActivityRecordTests#testCreateRemoveLegacySplashScreenWindow'
Flag: EXEMPT TEST_ONLY
Change-Id: Ie36addb0bccc0b5150c09b0e4372a3f683129c32