firestore: re-write BackgroundQueue to NOT use the deprecated AsyncTask thread pool#7376

MarkDuckworth merged 7 commits intomainfirebase/firebase-android-sdk:mainfrom

dconeybe/firestore/BackgroundQueueNoMoreAsyncTaskThreadPoolfirebase/firebase-android-sdk:dconeybe/firestore/BackgroundQueueNoMoreAsyncTaskThreadPoolCopy head branch name to clipboard