resourcemanager: fix a leak with death notifier

- The cookie object is deleted by setting up
AIBinder_DeathRecipient_setOnUnlinked
- Since calling std::weak_from_this from a constructor
is undefined, moving it out of the constructor.

Bug: 295395495
Test: atest android.media.misc.cts.ResourceManagerTest
      atest android.media.misc.cts.ResourceManagerMultiTest
      /data/nativetest64/ResourceManagerService_test/ResourceManagerService_test
      /data/nativetest64/ResourceObserverService_test/ResourceObserverService_test
Change-Id: I91017a021c6eb9fc4c1e5b04780814dd7550abbf