Fix DeathRecipient not work problem for UnsolicitedEventListener

linkToDeath of binder only use weak pointer ref for deathRecipient.
Hence local variable deathRecipient would destruct immediately after
registerUnsolEventListener function done.
Use a map to store all deathRecipient.

Bug: 129370555
Test: built, flashed, booted
          system/netd/tests/runtests.sh pass

Change-Id: I1829222dc071e237974033559bfbcce72b05d3fb