Don't tag bugreport notifications as System.

Otherwise, they'll skip DND settings.

Test: mmm -j32 frameworks/base/packages/Shell && adb install -r -g ${OUT}/data/app/ShellTests/ShellTests.apk && adb shell am instrument -e class com.android.shell.BugreportReceiverTest$1 -w com.android.shell.tests/android.support.test.runner.AndroidJUnitRunner

Bug: 63509844

Change-Id: I7d802b4c8d115c989979a9edaff2f83fc19a4eae