Cleanup public exposure of createEncoded{Audio|Video}Streams methods

Also group together the methods for Audio transforms and Video
transforms while we're here.

Change-Id: I7f16fbc2f1070f26734b792bcef5c1ad002147f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5428093


Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Auto-Submit: Tony Herre <toprice@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283133}