Add Android.mk to aid integration with FirefoxOS

This Android.mk is part of "Bug 831147 - Integrate apitrace into build"
https://bugzilla.mozilla.org/831147 With this addition, when apitrace
sources are cloned into B2GROOT/external/apitrace, then the FirefoxOS
build system picks up automatically apitrace, compiles and installs it
to the right place.

Signed-off-by: José Fonseca <jose.r.fonseca@gmail.com>
Set tabstops to 8