libbinder_ndk: association: rem heap allocs

libbinder_ndk, in the implementation of AIBinder_associateClass made
copies of and converted both IBinder and AIBinder_Class interface
descriptors to String8, even though they are already String16.

Bug: N/A
Test: atest aidl_integration_test
Change-Id: I99a5ed1dd06d32bfd28821a16792f37163b5179d