Add support for @SystemApi annotation to aapt.

When aapt sees @SystemApi in the XML comment associated with a resource,
it will now remove this tag from the comment (since is it not meaningful as
a Javadoc tag) and instead add an @android.annotation.SystemApi annotation
to the declared type or member.  This enables easy downstream processing of
this annotation from generated code.

Change-Id: If984908477648c0c90be52fdc955fbb891d99b28