Bug 41304: Add Android-specific targets to makefiles

Add Android-specific targets to makefiles (only fat-aar for the moment).

Also, added a mozconfig file to use for this.

We might think to a better way to handle this, because we might need to customize the mozconfig we use like we do on desktop.

For example, in my mozconfig I have:

ac_add_options --with-java-bin-path=/usr/lib/jvm/java-11-openjdk-amd64/bin
ac_add_options --with-android-sdk=/home/piero/Tor/android-toolchain/android-sdk-linux

I think we could add a non-versioned shell file that is sourced by the various script and adds some local config variables.

Another idea is not versioning the -dev android file, but creating it only locally (and remember to add the default channel setting).

Fixes #41304 (closed).