Make licenses info untranslatable

Mark the licenses dialog as untranslatable.

Unsure if this does anything in the normal build, but if Conversations ever decides to use a web translating service or if you're using the Android Studio translations editor this prevents you from adding a translation (it may also make the linter complain / cause the build to fail if someone else tries to add a translation later without realizing that you don't want to translate this sort of thing, which is good).