Convert Android documentation from regular txt format to Sphinx (RST).
Also add Android index.rst file and reference it in root index.rst, so
that Android documentation is visible.

Test:

    $ make htmldocs
    $ xdg-open doc/output/index.html
Signed-off-by: NSam Protsenko <joe.skb7@gmail.com>
Signed-off-by: NLokesh Vutla <lokeshvutla@ti.com>