Silence progress notifications

- First, we lower the importance of default channel.
- Then, we set setOnlyAlertOnce of progress notifications to true.
- Also, move the logic to create channel to the place where we construct
  progress notification for the first time only (instead of creating the
  channel every time when we construct a progress notification). This
  didn't lead to a bug but it should be good to change it.

Bug: 37217739

Test:
1. manually verified b/36530928 doesn't happen with this CL
2. manually verified no sound is heard when importing/exporting
   contacts, even if changing the importance of the default channel to
   make sound.

Change-Id: I9a6d1e2f6dae78ac518589cb07721b15a382445e