From the log of the build:
Caused by: java.lang.IllegalArgumentException: Keystore file not set for signing config release
Are you sure that you reverted the build.gradle changes (the ones mentioned here http://devcenter.bitrise.io/android/code-signing/#1-specify-code-sign-configuration-in-your-project )? It seems your build.gradle is still configured to use a keystore file.
I’d suggest you to remove the signingConfigs section from there (e.g. https://github.com/bitrise-samples/sample-apps-android-sdk22/blob/master/app/build.gradle); that should allow gradle to generate an unsigned release APK, which then can be signed by the Sign APK step.
Let me know how it goes or if you’d have any questions! 