Set RBE_DIR default value to point to prebuilts in tree.

The new default points to prebuilts/remoteexecution-client/live.

Also sets the default exec strategy of Javac/R8/D8 to remote local
fallback.

Test: simple one action build
Change-Id: I3392a0f7ac6de818d0c5ba2e113fcb035c6fa0ae
diff --git a/core/rbe.mk b/core/rbe.mk
index f4f7f5e..5e55cfb 100644
--- a/core/rbe.mk
+++ b/core/rbe.mk
@@ -19,7 +19,7 @@
   ifdef RBE_DIR
     rbe_dir := $(RBE_DIR)
   else
-    rbe_dir := $(HOME)/rbe
+    rbe_dir := prebuilts/remoteexecution-client/live/
   endif
 
   ifdef RBE_CXX_EXEC_STRATEGY
@@ -37,19 +37,19 @@
   ifdef RBE_JAVAC_EXEC_STRATEGY
     javac_exec_strategy := $(RBE_JAVAC_EXEC_STRATEGY)
   else
-    javac_exec_strategy := local
+    javac_exec_strategy := remote_local_fallback
   endif
 
   ifdef RBE_R8_EXEC_STRATEGY
     r8_exec_strategy := $(RBE_R8_EXEC_STRATEGY)
   else
-    r8_exec_strategy := local
+    r8_exec_strategy := remote_local_fallback
   endif
 
   ifdef RBE_D8_EXEC_STRATEGY
     d8_exec_strategy := $(RBE_D8_EXEC_STRATEGY)
   else
-    d8_exec_strategy := local
+    d8_exec_strategy := remote_local_fallback
   endif
 
   platform := container-image=docker://gcr.io/androidbuild-re-dockerimage/android-build-remoteexec-image@sha256:582efb38f0c229ea39952fff9e132ccbe183e14869b39888010dacf56b360d62