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