[Non-CMA] Fix the logic for deciding when to use SbDrmSystem.

This is the non-CMA equivalent of crrev.com/c/6762155. It fixes a bug
when casting Paramount+ or Peacock, in the scenario where ads play
before DRM-protected content.

Ideally we would could always pass a non-null SbDrmSystem to
SbPlayerCreate, but some partner impls cannot handle that scenario when
software decoding is used (e.g. for VP8).

Bug: b:432142194
Change-Id: Ie2c20260844db96ff37ede1b9e636210fb4afa42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6799125
Reviewed-by: Shawn Quereshi <shawnq@google.com>
Commit-Queue: Antonio Rivera <antoniori@google.com>
Cr-Commit-Position: refs/heads/main@{#1495050}