Provide warning about using "allowBackup" with Hilt

[pablobaxter]

Currently, there's a known issue with android:allowBackup=true and having a subclassed Application in that if a service or receiver is triggered while the app is backing app and attempts to inject a context, it could potentially crash. A side effect is that a crash while the app is backing up will leave it in the "restricted" mode that disabled the subclassed Application, and the next app start (either user initiated or via intent) will cause injection to fail once again.

Google bug ticket describing this: https://issuetracker.google.com/issues/160946170

Given that Hilt requires we subclass Application, there should also be a warning about using backup on the app as well.

[bcorso]

@pablobaxter do you have a recommendation for where you would want this information to be located, e.g. javadoc of some class/annotation vs somewhere in the dagger.dev/hilt documentation?

[pablobaxter]

A few places, for example:

• The @HiltAndroidApp doc
• In the documentation
  • https://dagger.dev/hilt/application and https://dagger.dev/hilt/quick-start. Both of these state that in order to use Hilt, you must override the Application and use the @HiltAndroidApp
• In the README.md

Basically, anywhere where it states that the Application context must be overridden, a warning about allowBackup should be placed as well. For reference as to why I'm making this suggestion, refer to this github issue: #748

[Chang-Eric]

We discussed this and I think maybe a known issues or FAQ section on the site will be good for longstanding issues like this.

[pablobaxter]

Both of those options are great places for this known issue, but a concern I have is that these crashes and issues are not always obvious as to what is the root cause. I spent some months to narrow an issue down to it being because of me overriding the Application class along with usage of allowBackup (which defaults to true). During my debugging, the issue kept pointing me back to either Dagger, Firebase, or some other areas which made the problem non-obvious.

Having some sort of note about overriding Application and using allowBackup upfront could have potentially cut down on my investigation and saved me much heartache. In all honesty, this note should be in the Application documentation as well, but that's a different issue altogether.

[aberaud]

We just found this issue after years of trying to understand Play Console reports that our app was crashing with
Caused by java.lang.IllegalStateException: Hilt service must be attached to an @HiltAndroidApp Application. Found: class android.app.Application.

Not being able to use allowBackup with Hilt is a major limitation that should be documented, especially since allowBackup is true by default. Ideally the build should fail in that scenario.