Bugfix: PIPELINE_DECODE_ERROR when switching from Clear to DRM content

[hongfeih-es]

This PR will...

Fix PIPELINE_DECODE_ERROR when switching from clear content to DRM content.

Why is this Pull Request needed?

When play DRM streams start with clear contents, player will report error like

The video playback was aborted due to a corruption problem or because the video used features your browser did not support - PIPELINE_ERROR_DECODE: audio decoder reinitialization failed

setMediaKeys need to be called before encrypted segments push to buffer

Are there any points in the code the reviewer needs to double check?

No

Resolves issues:

#4230

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[robwalch]

This should be handled by #4930. If you're still seeing the issue and mediaKeys are not reset on key change, could you share a stream sample that reproduces the issue?