Implement name() and flags() in IMediaExtractor

These were always present in the interface, but not actually implemented.
Now name and flags are passed through the Binder interface to the client.
The name() method was changed to String8 from "const char *" to avoid
ambiguity regarding who owns the string.

Bug: 143263164
Test: manual
Change-Id: I503d964b70f0affdedba23aa7da530164c14caf1