[DocsUI Search]: Adds ability to have Kotlin unit tests.

All new code is supposed to use Kotlin. I presume this means unit tests,
too.

Test: m DocumentsUIGoogle
Bug: 378590632
Change-Id: Ibf6dae1171e6d2438b4e726efaedcc7224ed1c20
Flag: EXEMPT enabling Kotlin code in tests; build file change only