TestEngineering/Performance/NimbleDroid

As of around May 2020, we no longer use Nimbledroid: https://github.com/mozilla-mobile/fenix/issues/11050

We've switched to an in-house system instead (FNPRMS, to be replaced by mach perftest) because: - it's difficult to debug performance when working with a black box

- Nimbledroid tests run with

debuggable=true

and that provides unreliable performance characteristics