1. Remove legacy screen-saver-blocking logic
2. tls_index_ is not a good choice, we can use thread-static
3. ScreenCapturerHelper is not designed for this scenario
4. Disable this capturer on 2+ monitors system
BUG=638802
Review-Url: https://codereview.webrtc.org/2319383002
Cr-Commit-Position: refs/heads/master@{#14342}