Avoid calling into MediaProvider from DownloadProvider.onCreate().

- Postpone marking few potential files in mediastore as downloads
  by reusing the MediaScanTriggerJob.
- Take care of deleting/orphaning downloads belonging to uninstalled
  packages upon receiving MEDIA_MOUNTED broadcast. This should also
  prevent DownloadProvider from accessing external storage before
  it is available.

Fixes: 140200277
Fixes: 154350921
Test: atest tests/app/src/android/app/cts/DownloadManagerTest.java
Test: manual

Change-Id: I57a05495b6aed3443c20ca3ca3c8a06ae87f3748
5 files changed