Add dependency verification and remove information leak from OkHttp

This MR adds dependency verification: https://docs.gradle.org/current/userguide/dependency_verification.html

It also removes the User Agent header from OkHttp and replaces it with our own ("PixelDroid"). An open question is if we should include version information in that header (for example "PixelDroid v1.0beta16") or if it is better not to