Move buildSrc classes to androidx.build package.

Test: ./gradlew assembleDebug for support and flatfoot projects
Change-Id: I9ecada2082209e46058dbce453d7222d3999c925
diff --git a/preference-leanback/build.gradle b/preference-leanback/build.gradle
index fc88f28..e5c19da 100644
--- a/preference-leanback/build.gradle
+++ b/preference-leanback/build.gradle
@@ -1,5 +1,5 @@
-import android.support.LibraryGroups
-import android.support.LibraryVersions
+import androidx.build.LibraryGroups
+import androidx.build.LibraryVersions
 
 plugins {
     id("SupportAndroidLibraryPlugin")