fix: when expression must be exhaustive by adding else branch

[hurali97]

Fixes issue #106

Description of changes:

In Kotlin 1.6.0, non-exhaustive when was marked as a compile-time warning, if used as a statement. In 1.7.0, a compile-time error is thrown to make this behaviour consistent with when expressions. More info here

For our use case, we added an empty else branch to the when statement on Player.State.

To test this, add kotlinVersion= "1.7.0" to android/build.gradle to the example project, like so:

buildscript {
    ext {
        buildToolsVersion = "30.0.2"
        minSdkVersion = 21
        compileSdkVersion = 30
        targetSdkVersion = 30
        ndkVersion = "21.4.7075529"
        kotlinVersion= "1.7.0"
    }
 // ...other stuff
}

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[maxstoller]

@alexmdodge @david-pw Should this be merged? Thanks!

[dawhitla]

Let's go ahead and get this merged @maxstoller