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.

java_common.gradle

@@ -115,6 +115,7 @@ spotless {
   format 'misc', {
     clearSteps()
     target '**/*.gradle'
+    targetExclude '**/cloudbuild-caches/**'
     trimTrailingWhitespace()
     indentWithSpaces(2)
     endWithNewline()