Make Gradle dependency cache shareable in GCB (#479)

* Make Gradle dependency cache shareable in GCB

Make Gradle put its caches in the source tree so that
they can be preserved across steps. When left at their
default location, caches are lost after each step.

config/presubmits.py

@@ -22,7 +22,7 @@ import sys
 import re
 
 # We should never analyze any generated files
-UNIVERSALLY_SKIPPED_PATTERNS = {"/build/", "/out/"}
+UNIVERSALLY_SKIPPED_PATTERNS = {"/build/", "cloudbuild-caches", "/out/"}
 # We can't rely on CI to have the Enum package installed so we do this instead.
 FORBIDDEN = 1
 REQUIRED = 2