Add missing nullability annotations to navigation

Enable lint check to ensure public API is fully annotated.

Test: ./gradlew :na:na-co:linD :na:na-fr:linD :na:na-ru:linD :na:na-ui:linD :na:na-te:linD
Change-Id: I87b408e844e16e040ff6b1c2431aa0da332132b9

diff --git a/navigation/runtime/build.gradle b/navigation/runtime/build.gradle
index 9b8cd0e..ff62b99 100644
--- a/navigation/runtime/build.gradle
+++ b/navigation/runtime/build.gradle

@@ -23,6 +23,12 @@
     id("SupportAndroidLibraryPlugin")
 }
 
+android {
+    lintOptions {
+        fatal("UnknownNullness")
+    }
+}
+
 dependencies {
     api(NAV_SUPPORT_CORE_UTILS)
     api(project(":navigation:navigation-common"))