Add cues to CueGroup

[ArchangelWTF]

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

Fixes https://github.com/Baseflow/ExoPlayerXamarin/issues/151

: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 Could you accept this PR and update the version (Create a release?)

[isaranghi]

Something is wrong... cueGroup.Cues() is crashing with newer nuget package.

[isaranghi]

But when I am trying to call the cues using reflection it works...

var javaCues = cueGroup.Class.GetDeclaredField("cues") .Get(cueGroup) as AbstractCollection;