Add Google-style ReadFileToString and WriteStringToFile.

Bug: 19217569
Change-Id: I64789d3fa6777ee30a324cdf00fdc275c1230e1b
diff --git a/libutils/tests/Android.mk b/libutils/tests/Android.mk
index 634f44f..03d5342 100644
--- a/libutils/tests/Android.mk
+++ b/libutils/tests/Android.mk
@@ -26,6 +26,7 @@
     BasicHashtable_test.cpp \
     BlobCache_test.cpp \
     BitSet_test.cpp \
+    file_test.cpp \
     Looper_test.cpp \
     LruCache_test.cpp \
     String8_test.cpp \