Just build the sdremote app for now. Note that this is a pure Java
app with no dependencies on (native) code (or Java code, for that
matter) from rest of LO.

Probably should drop the separate android/sdremote/Makfile and just do
what it does in android/CustomTarget_sdremote.mk instead.

Adding other Android apps (well, the LibreOffice4Android one likely)
to gbuild will require more complexity as they bundle native code, and
thus should depend on other modules first having been built. If one
wants to go really fancy, one could of course depend on the specific
libraries (and other files) being bundled. Let's see...

Change-Id: If10761479f348c4993eec40b7f8346edb77f0e0d