Write mkdirs in more idiomatic C++ style.

~ Rewrote mkdirs to be in C++ style.
~ Replaced adb_dir{start,stop} with std::string params and (r)find.
+ Added test for mkdirs.

Also make base/test_utils.h public and support temporary directories
as well as files.

Change-Id: I6fcbdc5e0099f3359d3aac6b00c436f250ca1329
diff --git a/base/Android.mk b/base/Android.mk
index 7bd317b..4e135f6 100644
--- a/base/Android.mk
+++ b/base/Android.mk
@@ -21,6 +21,7 @@
     logging.cpp \
     stringprintf.cpp \
     strings.cpp \
+    test_utils.cpp \
 
 libbase_test_src_files := \
     file_test.cpp \
@@ -28,7 +29,6 @@
     stringprintf_test.cpp \
     strings_test.cpp \
     test_main.cpp \
-    test_utils.cpp \
 
 libbase_cppflags := \
     -Wall \