libmediatranscoding: handle death recipient cookie ownership differently

The ownership of the death recipient cookie is now limited to the
TranscodingResourcePolicy object and the binderDied callback.
They both must be able to delete the cookie object and they both must be
aware of it already being deleted.

In all cases, the TranscodingResourcePolicy object that needs to be
unregistered will outlive the cookie and the death recipient.

Calling unlinkToDeath is unneccessary because the last strong ref to the
binder that was linked to death is removed in the unregisterSelf method
which will unlink the binder and death recipient.

Test: atest CtsMediaTranscodingTestCases MediaSampleReaderNDKTests
Test: adb shell kill -9 `pid  media.resource_observer`
Test: delete mResourcePolicy.get() to force destructor after linkToDeath
Bug: 319210610
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:0c674f5ff68daa64b90e1a234061ba9bebe6173c)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:0213a7d62ea5545b954145cc53e2bd65ed5dc609)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:686987b9613678fe6540079e85e0d94519551738)
Merged-In: I8e6ba40fe3da30bf8753e7a16ad5c8cd5dfda40b
Change-Id: I8e6ba40fe3da30bf8753e7a16ad5c8cd5dfda40b