add ktlint plugin to project and apply default code style (#2209)

* add ktlint plugin to project and apply default code style

* some manual adjustments, fix wildcard imports

* update CONTRIBUTING.md

* fix formatting

app/src/test/java/android/text/SpannableString.kt

@@ -23,7 +23,6 @@ class SpannableString(private val text: CharSequence) : Spannable {
     override val length: Int
         get() = text.length
 
-
     override fun nextSpanTransition(start: Int, limit: Int, type: Class<*>?): Int {
         throw NotImplementedError()
     }
@@ -47,4 +46,4 @@ class SpannableString(private val text: CharSequence) : Spannable {
     override fun getSpanStart(tag: Any?): Int {
         throw NotImplementedError()
     }
-}
+}