Move stringpiece.h and stringprintf.h to base/.

Change-Id: I7f71b4a12f99c5f81771146c66629ae5a947b229
diff --git a/build/Android.common.mk b/build/Android.common.mk
index 03d403f..9bc7541 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -132,6 +132,8 @@
 	src/atomic.cc.arm \
 	src/base/logging.cc \
 	src/base/mutex.cc \
+	src/base/stringpiece.cc \
+	src/base/stringprintf.cc \
 	src/base/unix_file/fd_file.cc \
 	src/base/unix_file/mapped_file.cc \
 	src/base/unix_file/null_file.cc \
@@ -227,8 +229,6 @@
 	src/runtime_support.cc \
 	src/signal_catcher.cc \
 	src/stack.cc \
-	src/stringpiece.cc \
-	src/stringprintf.cc \
 	src/thread.cc \
 	src/thread_list.cc \
 	src/thread_pool.cc \