recovery: Switch to clang

And a few trival fixes to suppress warnings.

Change-Id: Id28e3581aaca4bda59826afa80c0c1cdfb0442fc
(cherry picked from commit 80e46e08de5f65702fa7f7cd3ef83f905d919bbc)
diff --git a/minui/Android.mk b/minui/Android.mk
index 97724fb..3057f45 100644
--- a/minui/Android.mk
+++ b/minui/Android.mk
@@ -41,6 +41,7 @@
 
 # Used by OEMs for factory test images.
 include $(CLEAR_VARS)
+LOCAL_CLANG := true
 LOCAL_MODULE := libminui
 LOCAL_WHOLE_STATIC_LIBRARIES += libminui
 LOCAL_SHARED_LIBRARIES := libpng