Don't attach PendingIntent if settings app is hidden.

The settings app may not be available. In that case, just show the adb
notification without a PendingIntent.

Bug: 156453114

Test: atest AdbNotificationsTest
Test: With USB debugging enabled, install TestDPC, and use it to hide settings app.
Unplug and replug USB. USB debugging notification shows up and clicking
it does nothing.
Change-Id: Ie29d2c425c05bce9371600d76e4eb2eaba692fd7

Change-Id: Ie5f746cbc7b8a32fc280177bf281a9e973c8df12