fix proguard when gradle android plugin allows for proguard.cfg use

[AEtherSurfer]

Once this fix
jvoegele/gradle-android-plugin@90df7bb
to the android plugin makes it to release, update the build.gradle to that version and make the neccesary changes to the proguard.cfg file.

Known proguard issues:

• gson.fromJSON() in OutpointService
• refreshBanlance call from Refresh menu in Bitcoin.java