Hold onto linked binder.

unlinkToDeath is no longer required, (this change in behavior is to
avoid leaks) so holding onto ActivityManager here (still calling
unlinkToDeath to avoid a log, but may for instance remove all
unlinkToDeath calls in the future).

Exempt-From-Owner-Approval: approval received on AOSP CL, only
    whitespace difference here.

Bug: 134576445
Test: boot
Change-Id: I273f77aac2b80ba9be70197cc3842f83a11bbd1c