[Firebase_Database] Fix possible NullPointerException

[juliocbcotta]

Description

This PR fixes a possible NullPointerException that happens when the plugin is registered without a valid Activity.
I took the opportunity to add some nullability annotations that the IDE was suggesting, those are in the interfaces that are being implemented in the plugin, so should be safe to use.

Related Issues

#19

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[audkar]

@BugsBunnyBR @collinjackson isn't this PR changes FirebaseDatabase plugin behavior contract

from:

Always emit events on Main thread

to:

Always emit events on thread which on which plugin was instantiated

To be compatible with prievious behaviour this should be changed:

private final Handler handler = new Handler();

to this:

Handler handler = new Handler(Looper.getMainLooper());

?

[juliocbcotta]

@audkar, Indeed this would break things, but when is plugin registration happening in any other thread than Main Thread? I think that registering plugins in any thread other than the Main Thread is a mistake and plugins should break if that happens as there is no support to do so in Flutter. (JNI stuff needs to happen in Main Thread as far as I know)

[audkar]

I manually create&register plugins in my app context :)

Of course I do that on main thread, but still I if would make mistake, than previous implementation would work and current not.

[collinjackson]

I don't feel strongly either way. I would be ok with @audkar's Looper.getMainLooper() approach, or we could enforce that we're on the main thread during plugin registration.