Change sendUnsentReports return type to async

[acntwww]

Change FirebaseCrashlytics#sendUnsentReports return type to Task<Void>

With a Task type result returned, we can hold and wait the task to be finished if we want.

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[thatfiredev]

@acntwww I'm curious about your use-case. Why would you want to await on the result of this?

[acntwww]

@thatfiredev I init the firebase sdk all by hand with other SDKs , saying sdk-a.

private void init() {
  initFirebase();
  initSdkA(); <--- this is where the crash happens
}

This crash may not be uploaded. So I try to upload the crash info in initFirebase function, like:

private void initFirebase() {
  ...
  Task<Void> unsentReports = firebaseCrashlytics.sendUnsentReports();
  Utils.awaitEvenIfOnMainThread(unsentReports);
  ...
}