healthd: notify listeners using local copy of list, drop lock

Binder currently may service an incoming oneway transaction whenever
an outbound oneway call is made (if there is already a pending
incoming oneway call waiting).  The unexpected nested method call
blocks forever on a recursive mutex acquire because healthd is single-
threaded.  The binder behavior is considered a bug and may change in
the future.  For now, work around this in healthd.

Make a local copy of the listeners list, then drop the lock and
perform the outbound calls on the local copy of the list.

Bug: 38201220
Test: Marlin with modified client calling scheduleUpdate() repeatedly
Change-Id: If35c2847556245921e2aff808ff747bb60356811
(cherry picked from commit fd683700)