More volume UUID awareness.

Teach free_cache() and restorecon_data() about building per-volume
paths.  Also clean up restorecon_data() by using std::string when
building paths.

Clearer names for path building utility methods, and tests to verify.

Bug: 19993667
Change-Id: Iacfbcdaa5b901cc2490bc8eba366dfdeb44f1d93
diff --git a/cmds/installd/tests/Android.mk b/cmds/installd/tests/Android.mk
index c16375a..38a9f69 100644
--- a/cmds/installd/tests/Android.mk
+++ b/cmds/installd/tests/Android.mk
@@ -26,5 +26,6 @@
     $(eval LOCAL_SRC_FILES := $(file)) \
     $(eval LOCAL_C_INCLUDES := $(c_includes)) \
     $(eval LOCAL_MODULE := $(notdir $(file:%.cpp=%))) \
+    $(eval LOCAL_CLANG := true) \
     $(eval include $(BUILD_NATIVE_TEST)) \
 )