IssueTracker

Library used (Espresso, Runner, Rules etc.): android.arch.core:core-testing
Version used: 1.1.0

What steps will reproduce the problem?
1. Try to use ArchTaskExecutor directly, because of JUnit5

JUnit5 (supported via https://github.com/mannodermaus/android-junit5) doesn't support most `@Rule`s. It uses an extension model, instead. As such, I have a custom extension named `InstantTaskExecutorExtension`:

```
class InstantTaskExecutorExtension : BeforeEachCallback, AfterEachCallback {

    @SuppressLint("RestrictedApi")
    override fun beforeEach(context: ExtensionContext) {
        ArchTaskExecutor.getInstance().setDelegate(object : TaskExecutor() {
            override fun executeOnDiskIO(runnable: Runnable) {
                runnable.run()
            }

            override fun postToMainThread(runnable: Runnable) {
                runnable.run()
            }

            override fun isMainThread(): Boolean {
                return true
            }
        })
    }

    override fun afterEach(context: ExtensionContext?) {
        ArchTaskExecutor.getInstance().setDelegate(null)
    }
}
```

Used like:
```
@ExtendWith(InstantTaskExecutorExtension::class)
internal class AuthenticationInterceptorTest { ... }
```

Works great. However, I recently decided to move code like this to a common module so it can be consumed by more than just my `app` module. So I ended up putting it in the main sourceSet (so it can be consumed via `"testImplementation project(:junit5-extensions)"`. That's when Lint complained I was exceeding my access rights to `ArchTaskExecutor`. I can suppress it, of course, but this seems non-ideal. Why is that restriction on that class?