Enable opening mailto URLs under Android

Enable opening mailto URLs under Android

* doc/emacs/android.texi (Android Startup): Mention how mailto
URLs are treated by the emacsclient wrapper.

* java/AndroidManifest.xml.in: Register `mailto' scheme filters
for EmacsOpenActivity.

* java/org/gnu/emacs/EmacsOpenActivity.java (startEmacsClient):
Extract code that starts Emacs when it isn't already running,
and take a list of arguments rather than a single file name.
(onCreate): If the scheme is `mailto', escape the URI and call
`message-mailto'.