Validate ndkmediacodec callback pointers before use

Verify that the async callback function pointers are non-null before use.
Rework callback setting to allow retrigger after a release/configure cycle.

Bug: 205335443
Bug: 152553625
Bug: 203147387
Test: atest android.mediav2.cts.CodecUnitTest
Change-Id: I536575f280c714b6a702af3b48a410f6d9a7a924