Convert notifications to use NotificationChannel

Nofication Channels needs to be used to make sure that the priority of
the notifications are appropriate and they appear. This change was made
in O where notifications built without using wouldn't show up.

Bug: 38243950
Test: Send and receive files using Android Beam
Change-Id: I9e53c802eb9b9a1af108cb42b7424cda5be5762d
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>