stagefright: MediaCodecsXmlParser: add support for parsing rank

Rank is supplied in a limit:

<Limit name="rank" value="4" />

This overrides the rank provided by the component stores (C2Store,
or default values for OMX).

Bug: 119631295
Change-Id: I01cfcaf1dcdb60f8bd6f907ebec0ed3f4e3b117b