Allow explicit warnings for projects

Enabled compiler warnings by default and then modified javac
command to include warnings only if a project's warning flag,
LOCAL_WARNINGS_ENABLE, is set to true.

Change-Id: I5111d16a862f3afaaf1dcba2f2c2b7fc072613f1

core/clear_vars.mk

@@ -92,6 +92,7 @@ LOCAL_NO_EMMA_INSTRUMENT:=
 LOCAL_NO_EMMA_COMPILE:=
 LOCAL_PROGUARD_ENABLED:= # '',optonly,full,custom
 LOCAL_PROGUARD_FLAGS:=
+LOCAL_WARNINGS_ENABLE:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.

core/definitions.mk

@@ -1238,7 +1238,7 @@ endef
 ifeq ($(HOST_OS),windows)
 xlint_unchecked :=
 else
-#xlint_unchecked := -Xlint:unchecked
+xlint_unchecked := -Xlint:unchecked
 endif
 
 # emit-line, <word list>, <output file>
@@ -1308,7 +1308,8 @@ $(hide) tr ' ' '\n' < $(dir $(PRIVATE_CLASS_INTERMEDIATES_DIR))/java-source-list
 $(hide) $(TARGET_JAVAC) -encoding ascii $(PRIVATE_BOOTCLASSPATH) \
     $(addprefix -classpath ,$(strip \
         $(call normalize-path-list,$(PRIVATE_ALL_JAVA_LIBRARIES)))) \
-    $(PRIVATE_JAVACFLAGS) $(strip $(PRIVATE_JAVAC_DEBUG_FLAGS)) $(xlint_unchecked) \
+    $(PRIVATE_JAVACFLAGS) $(strip $(PRIVATE_JAVAC_DEBUG_FLAGS)) \
+	$(if $(findstring true,$(LOCAL_WARNINGS_ENABLE)),$(xlint_unchecked),) \
     -extdirs "" -d $(PRIVATE_CLASS_INTERMEDIATES_DIR) \
     \@$(dir $(PRIVATE_CLASS_INTERMEDIATES_DIR))/java-source-list-uniq \
     || ( rm -rf $(PRIVATE_CLASS_INTERMEDIATES_DIR) ; exit 41 )