DocsUI: Fix tests failing with NoSuchMethodError getForceMaterial3

The method gets optimized away when the flag is disabled, making some
tests fail.

Bug: 425725242
Test: atest com.android.documentsui.AbstractActionHandlerTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: I4911ed3ca331c469803affebf1a4ffa8d9ec529a