Revert "WifiService: Remove usage of NotificationChannel.setBlockableSystem"

This reverts commit 036351447a34340a82f23026bb564fa88e6dd03f.

Reason for revert: setBlockableSystem() was made a @SystemApi in b/138401571

Bug: 145921908
Change-Id: I94ed171bb08a244a67dd263a2e4bfb6cf23ce1f1
Test: Compiles
1 file changed
tree: cff8225525154d5a425d5bb739499c757d51d8e6
  1. apex/
  2. service/
  3. PREUPLOAD.cfg