[FR] ProGuard Rules Shouldn't Ignore Warnings?

Hello fantastic Firebase folks,

It looks like all the ProGuard specs start by ignoring all warnings for the project that uses them, with `-ignorewarnings`.
(To reproduce, run `grep ignorewarnings external/firebase_cpp/libs/android/*.pro` from the latest release.)

This seems...dangerous, and is pretty heavily discouraged by the [ProGuard Manual](https://www.guardsquare.com/manual/configuration/usage) and disallowed in Bazel for similar reasons.

Is it necessary? (Things seemed to work fine without in a quick test) And if not necessary, would you be open to removing `-ignorewarnings`?

Cheers,
Chris