I'm afraid that this cannot be done using the Android app. Because simply doesn't support any java script (using show hide layers).
The solution as you said is to create a separate txt for AM/PM. I know that sometimes especially without the '0' the space appears a bit large but there is no other way. Try to play with the alignment or put the ampm text on top or bottom of the clock for the best result.
Edit: The show hide is supported on the Android but only for images. But for a single image. Creating ampm images you will need 3 images - one script code, so I'm not sure if it will work..
_________________
...and remember: don't take life too seriously...My profile on Deviantart:
http://jimking.deviantart.com/