Fix optimize.proguard_flags_files typo

LOCAL_PROGUARD_FLAG_FILES should convert to
optimize.proguard_flags_files, not optimize.proguard_flag_files.

Test: m checkbuild
Change-Id: I1471afc006a2a3892af37d2cb73c51636f05d7f7
diff --git a/androidmk/cmd/androidmk/android.go b/androidmk/cmd/androidmk/android.go
index 37877c8..ed6a356 100644
--- a/androidmk/cmd/androidmk/android.go
+++ b/androidmk/cmd/androidmk/android.go
@@ -147,7 +147,7 @@
 			"LOCAL_ANNOTATION_PROCESSOR_CLASSES": "annotation_processor_classes",
 
 			"LOCAL_PROGUARD_FLAGS":      "optimize.proguard_flags",
-			"LOCAL_PROGUARD_FLAG_FILES": "optimize.proguard_flag_files",
+			"LOCAL_PROGUARD_FLAG_FILES": "optimize.proguard_flags_files",
 
 			// These will be rewritten to libs/static_libs by bpfix, after their presence is used to convert
 			// java_library_static to android_library.