android: Avoid IllegalStateException when importing certificates

android: Avoid IllegalStateException when importing certificates

When certificates are imported via Storage Access Framework we did handle
the selection directly in onActivityResult().  However, at that point the
activity might apparently not yet be resumed.  So committing
FragmentTransactions could result in IllegalStateExceptions due to the
potential state loss.  To avoid that we cache the returned URI and wait
until onPostResume() to make sure the activity's state is fully restored
before showing the confirmation dialog.