Fix issue #17829949: Don't kill Bluetooth service...

...even in extreme low memory condition

Bind to Bluetooth with BIND_IMPORTANT, so that it is allowed to
go to a higher oom adj level.

Fix some problems when this is done from a system or persistent
process, where this would go to a level that is *too* high.  Instead,
introduce a new oom adj level for it that is right below persistent.

Change-Id: I002bcc4accc36c8579c4cda161be7d2fba21ba17