[plugman] renaming GCM/ to gcm/ to make plugman work

for some reason there were two folders in src/android/com/plugins: `GCM/` and `gcm/`. On OSX doing an `ls` only shows `GCM` because OSX is case insensitive. But plugman and git see it differently, and plugman fails to find the files in GCM because it looks in gcm.