Use constant apk file name in Gradle build

We used to have the Gradle build use time stamps in the name.
Having changing apk names causes weird issues in the IDE.
Multiple people have seen cases where Android Studio complained
about the apk file path not being present.
("Local path doesn't exist.")

Also:
* The push option was printing: pushing branch 2.0-beta-2014-10-14
even though we're not creating any branches anymore.
* Also updated the release script to do the renaming of the apk file name
  and to update the docs

Change-Id: Ib4395e52760c5a5b8f79ffe24edd662572d02efe