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 install is to the right place.