DocsUI M3: Add test rule to set force_material3

The test rule is to simplify the testing using the use_material3 flag,
forcing the config `force_material3` to be in sync with the desired flag
value.

Bug: 403375773
Test: atest DocumentsUIGoogleTests:com.android.documentsui.util.ThemeUtilsTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: Id951da2f9b929ed6f1a94cf6fd886413b79b43de