Player config should override license servers from the manifest

[joeyparrish]

Have you read the FAQ and checked for duplicate open issues?
Yes

What version of Shaka Player are you using?
v2.5.0-beta3 and master

Can you reproduce the issue with our latest release version?
N/A - feature #484 didn't exist in v2.4.x

Can you reproduce the issue with the latest code from master?
Yes

Are you using the demo app or your own custom app?
Demo

What browser and OS are you using?
Chrome, OS N/A

What are the manifest and license server URIs?
No manifest URIs provided in the original feature discussion.
No manifest URIs provided by the internal partner reporting the issue.
Should be reproducible easily with unit tests.

What did you do?
1. Configure the Player with license server URI for Widevine
2. Load DASH content with ms:laurl elements containing a Widevine license server URI

1. Configure the Player with license server URI for Widevine only
2. Load DASH content with ms:laurl elements containing both Widevine and PlayReady license server URIs

What did you expect to happen?
The Player config should override the manifest's license server, as discussed in the original bug report: #484 (comment)
The Player config should override the manifest's license servers, including those not specified in the config. So, for example, if the player config contains Widevine only, only Widevine should be used, in spite of PlayReady license server in the manifest.

What actually happened?
PR #1644 which added this feature did not handle the nuance of letting the player config take precedence. So the player config for license servers is ignored with such content.
Player config only replaced the manifest servers for Widevine, and did not erase the PlayReady license server. The app developer only wants Widevine to be used in this case.

[joeyparrish]

Unable to reproduce with manual hacks (simulating license server URI in manifest). I will go back to our partner for a manifest we can use to reproduce the issue.

[joeyparrish]

We have found the specific circumstances needed to reproduce this. The issue is not what it seemed.

1. A platform with both PlayReady and Widevine, such as Chromecast or other CE device
2. Content with ms:pro elements containing license servers for both PlayReady and Widevine, in that order
3. Player config with a license server for Widevine only

The player config overrides the manifest config only for the specified key system (Widevine), but doesn't erase the PlayReady license server derived from the manifest. Because the <ContentProtection> element for PR comes first, Shaka chooses that. The app's intent in only configuring Widevine was to force the use of Widevine instead of PlayReady.

I think it would be a reasonable change in behavior for the extracting license server URIs from the manifest to be used only if none are configured at the player level.