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

[AEtherSurfer]

Once this fix https://github.com/jvoegele/gradle-android-plugin/commit/90df7bb0048cd96b38a8d556132b91fc65f6858d 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