Add '-Werror' for main build and suppress one deprecation warning

[ghost]

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[ghost]

@rsgowman The tests failed, but I cannot reproduce locally. If you know on top of your mind why this fails please let me know.

[rsgowman]

The tests failed, but I cannot reproduce locally. If you know on top of your mind why this fails please let me know.

Try merging master. The first error I see here is:

/home/prow/go/src/github.com/firebase/firebase-android-sdk/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/FirestoreCallCredentials.java:21: warning: [deprecation] CallCredentials2 in io.grpc has been deprecated
import io.grpc.CallCredentials2;
^
error: warnings found and -Werror specified

grpc was recently updated in master (by me), so it's entirely possible that grpc has deprecated some features between the old version and the current one. Ideally, I would've noticed this while doing the update, but of course didn't, since we didn't have -Werror enabled. This is exactly the sort of thing we'll be able to avoid thanks to your change. :)

As to what to do here, https://grpc.github.io/grpc-java/javadoc/io/grpc/CallCredentials2.html says Deprecated. the new interface has been promoted into CallCredentials. Implementations should switch back to "extends CallCredentials".
so I think we can just s/CallCredentials2/CallCredentials/g within FirestoreCallCredentials.java.

Aside from our test suite, it would likely be worthwhile to run a quick manual test to ensure everything's still fine. This class deals with auth tokens, so something that logs in/out would be a good candidate. FriendlyEats (https://github.com/firebase/friendlyeats-android) should do it. If you're unsure of how to go about this, I can walk you through it.

[ghost]

Fixed all warnings and also run fireeats against this PR, it seems to work fine.