Using JSON service account credentials

[domdomegg]

What

PKCS (.p12) service account credentials have been deprecated in the Triple-T/gradle-play-publisher plugin for over a year now, and support has finally been dropped in version 3. We should migrate to JSON service account credentials instead.

How

I think to do this we need to:

1. Login to the GCP project where the current service account is
2. Generate JSON credentials for the account
3. Encrypt them with the secret key and pop them in the repo
4. Change the Travis and Gradle build settings to decrypt and use the JSON credentials instead of play.p12

Steps 1 and 2 need to be done by someone with access to the GCP project, and step 3 needs someone with the encryption key. I'm not sure who has what access, and I don't think I have either. @misaochan @maskaravivek do either of you know?

Why

Removes a deprecation warning, and will allow us to update to version 3 of the plugin (see #4089), allowing us to update the Android Gradle Build Tools version (see #4088).