[native] Get rid of mediaProcessConfig and pass processMedia config directly

Summary:
Originally, I just moved the definition of onTranscodingProgress outside of the mediaProcessConfig function: D7475... but on second thought I think we can remove mediaProcessConfig altogether.

Constructing the MediaProcessConfig directly is a lot simpler and removes unnecesary indirection.

(I'm NOT planning any other InputStateContainer refactors. Noticed this because I was looking specifically at processMedia for image avatar work and it was a 2min change)

Test Plan:
flow/careful reading

transcoding progress appears correctly:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7477