Transcoder: Don't preserve profile/level when switching codec.

Avoid configuring encoder with source file's profile and level
if the codec type itself will change. Instead leave the default
unspecified. The caller can still set profile/level when
configuring the transcoder.

Bug: 182576621
Test: Transcoder unit tests.
Change-Id: I469a7a397ebf3e2eade6a48d95851656157578b7