Add LOCAL_FINDBUGS_FLAGS to support custom findbugs flags.

Use LOCAL_FINDBUGS_FLAGS to specify findbugs command line flags.
To use findbugs filters:
LOCAL_FINDBUGS_FLAGS := -exclude $(LOCAL_PATH)/exclude.xml -include \
                        $(LOCAL_PATH)/include.xml

where exclude.xml and include.xml are filter files.
The build system automatically adds the filter files as dependency of
findbugs' output xml.

Bug: 24948760
Change-Id: I82502c0ade910c5255001fa5f390316137ce0be6
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 5acb887..91dfdac 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -191,6 +191,7 @@
 LOCAL_DBUS_PROXY_PREFIX:=
 LOCAL_INIT_RC:=
 LOCAL_MODULE_HOST_OS:=
+LOCAL_FINDBUGS_FLAGS:=
 
 # arch specific variables
 LOCAL_SRC_FILES_$(TARGET_ARCH):=