Don't prepend ccache if it's already there

Bug: 3069576
Change-Id: I8345209c5c885ed7ed8e043e051884d64008961c
diff --git a/Makefile.android b/Makefile.android
index c057585..c9ed595 100644
--- a/Makefile.android
+++ b/Makefile.android
@@ -86,7 +86,11 @@
 ifneq ($(BUILD_STANDALONE_EMULATOR),true)
 
   ifneq ($(USE_CCACHE),)
-    MY_CC := prebuilt/$(HOST_PREBUILT_TAG)/ccache/ccache $(MY_CC)
+    ccache := prebuilt/$(HOST_PREBUILT_TAG)/ccache/ccache
+    ifneq ($(ccache),$(firstword $(MY_CC)))
+      MY_CC := $(ccache) $(MY_CC)
+    endif
+    ccache :=
   endif
 endif