SystemUI: Fix Removed USB Storage Notification

If you have connected a OTG USB Storage to your device an disconnect it
the Notification "Removed USB Storage" appears. But you can't swipe it
off, because the Notification intent is set with FLAG_ONGOING_EVENT and
not with FLAG_AUTO_CANCEL!
In my fix I changed the boolean function variable 'dismissable' from
"false" to "true" and so the Notofication will gets the flag
FLAG_AUTO_CANCEL, now the Notification can be swiped off.

Bug: https://paranoidandroid.atlassian.net/browse/AOSPA-191

Change-Id: I01713dbcbc025d0dd2df3e6d4359507e0e31a23f
Signed-off-by: Matthias Freund <matthias88freund@gmail.com>