Kotlin has several nice features for writing tests, such as lambdas and
default implementations for interface methods. This patch adds Kotlin
support to the geckoview module build.gradle. We don't want to use
Kotlin in non-test code yet, so the patch ensures that only test code
contains Kotlin files.

MozReview-Commit-ID: FcQiHj20xlB

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extra : rebase_source : e304d25d09291bc0a3faa29bf36f9d01eadc8524