Bump bindings up to ExoPlayer 2.18.3

[ArchangelWTF]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Fixes: https://github.com/Baseflow/ExoPlayerXamarin/issues/153

:arrow_heading_down: What is the current behavior?

:new: What is the new behavior (if this is a feature change)?

:boom: Does this PR introduce a breaking change?

:bug: Recommendations for testing

:memo: Links to relevant issues/docs

:thinking: Checklist before submitting

• [x] All projects build
• [x] Follows style guide lines
• [x] Relevant documentation was updated
• [x] Rebased onto current develop

[ArchangelWTF]

@martijn00 This seems ready to merge, I've build the sample app when creating the PR and there seems to be no regressions in functionality as far as I've noticed.

[martijn00]

Thanks!