Firebase Installations: throw exception on missing FIROptions.projectID or invalid API Key

[maksymmalyhin]

Background:

• FIROptions.projectID is needed for Firebase Installations SDK(FIS SDK) to be sent with API requests
• FIS SDK is a dependency for Firebase Instance ID SDK (IID SDK) (starting from IID SDK 4.3.0)
• IID SDK does not require FIROptions.projectID. It means that introducing a new dependency enforcing projectID to IID SDK technically is a breaking change and should not be introduced until a major release.

Workaround:
FIS SDK uses FIROptions. GCMSenderID in case if FIROptions.projectID is missing to avoid breaking changes. (see #4683)

To be done:

• Update FIS SDK to enforce FIROptions.projectID for the next major release (Firebase 7)
• Enforce API Key format to filter non-Firebase API keys accident use (b/147712985)

[ryanwilson]

Is there a downside to using the GCMSenderID on the server side? Just wondering if it's necessary to break eventually if GCMSenderID has no downsides.

[maksymmalyhin]

I think @andirayo can provide the best answer here. As far as I know using GCMSenderID for this purpose is a bit of a hack and is fine to use just until we can do breaking changes. Also GCMSenderID is considered to be deprecated, so we should promote using it.