Remove SBNHolder

This was added before heavy notif content was being put into
ashmem, and it means every notification update adds one binder call
per live NLS.

Test: NotificationManagerTest
Test: NotificationManagerServiceTest
Bug: 378128805
Flag: android.app.no_sbnholder
Change-Id: Ief60887b2e2067416978c298d81ef1edd4abd1e4