Use unified cas uploads / downloads

This is to deduplciate uploads and downloads when running builds with
RBE. Especially useful for certain Java / R8 / D8 actions that have JAR
files as common inputs.

Change-Id: I0459d4e5c89bd93e5c0c53195fdabccabec08674

rbesetup.sh

@@ -66,6 +66,7 @@ function _set_rbe_vars() {
   unset USE_GOMA
   export USE_RBE="true"
   export RBE_CXX_EXEC_STRATEGY="remote_local_fallback"
+  export RBE_use_unified_cas_ops="true"
   export RBE_JAVAC=1
   export RBE_R8=1
   export RBE_D8=1