Add tag "foo" to builder test's notifications.

This allows them to be conveniently cleared via adb:

  $ adb shell service call notification 5 \
    s16 com.android.statusbartest \
	s16 foo \
	i32 <NOTIFICATIONID> \
	i32 <USERID>

Bug: 8564511
Change-Id: I9fae5a4a0be64d6707cc06ca938adeb8b3c0b735