add an android cargo builder

[metajack]

We want some deps checked on android, but need a special cargo builder for that. There is already some work done in master.cfg so that specific projects can opt into android builds (ie, ANDROID_PROJECTS).

I think good candidates are any of the *-sys and their bindings: skia, rust-azure, mozjs, rust-mozjs, libfontconfig, etc. These were recently broken on android when trying to modify them for desktop arm linux systems. Some basic build checking would have prevented that bustage.

[larsbergstrom]

We're no longer doing Cargo builds.