Remove String8 and AMessage from extractor plugin interface.

- mime(String8): is dropped from the interface since it was only used
for debug message. debug message is replaced with MediaExtractor::name.
- meta(AMessage): is replaced with an opaque pointer, void *, in addition
to an additional function pointer to free the object.
(In MPEG4Extractor case, AMessage with "meta-data-size", which was
introduced in Id2acdde89, is removed since the use case had been gone
with AwesomePlayer.)

Test: cts-tradefed run cts-dev --module CtsMediaTestCases --compatibility:module-arg
CtsMediaTestCases:include-annotation:android.platform.test.annotations.RequiresDevice
Bug: 67908556
Change-Id: I242944332bf0ff7dd4ad2106fe8f68e97264010b