libbinder_ndk_unit_test: show cookie can own data

The existing example is dangerous because it gives the
death recipient ownership of data on the stack. If the
test fails, then the callback would be referencing
invalid stack data.

Now, this example owns the cookie, so it is more clear
how to have the cookie contain arbitrary additional
data, and there is no longer a memory corruption issue
if the test happened to fail.

Fixes: 275407810
Test: libbinder_ndk_unit_test

Change-Id: I2b2ce675d3fae7e033bd701208b095ebe14da1e5