Disable R8 for DocumentsUI

This failure seems related to R8:

https://sponge.corp.google.com/target?show=FAILED&sortBy=STATUS&id=4f1b303e-ef57-4dc6-9ebb-95ca5cda398e&target=com.android.documentsui.tests

Test: Run DucumentsUI tests, and the failure above is gone.
Change-Id: Ib04f5400fa78823cfc8b5294d247f08ffd184176
Merged-In: I828190c8742375abb1d20376010f203e45e0f0cf