libgui: Fix some bitrot/warnings in tests

These tests hadn't been run in a while and had accumulated some rot.
This fixes most of the compile warnings and some tests which fail
incorrectly, but 2 failures remain on N6.

Change-Id: I7fee78cd058a32f4d7df40ec9f899ff767f68517
diff --git a/libs/gui/tests/Android.mk b/libs/gui/tests/Android.mk
index 128a32a..6ad9986 100644
--- a/libs/gui/tests/Android.mk
+++ b/libs/gui/tests/Android.mk
@@ -3,6 +3,8 @@
 include $(CLEAR_VARS)
 LOCAL_ADDITIONAL_DEPENDENCIES := $(LOCAL_PATH)/Android.mk
 
+LOCAL_CLANG := true
+
 LOCAL_MODULE := libgui_test
 
 LOCAL_MODULE_TAGS := tests