DocsUI: Fix TestRule for Material3 flag to reset the original value

The test InflateMessageDocumentHolderTest seems to run as M3 even for
device configurations that don't have the flag enabled yet.

This CL tries to fix this by changing CheckAndForceMaterial3Flag to
restore the original flag result after the test has run, to avoid
interference between tests.

Bug: 424456315
Test: atest com.android.documentsui.util.ThemeUtilsTest
Test: atest com.android.documentsui.dirlist.InflateMessageDocumentHolderTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: I6902621a2e36561a1364a36cfaefd772b527ee06