Add a keep rule for AndroidDispatcherFactory for R8 version > 1.6.0 (K…

…otlin#2154)

In commit #3a8a0ea this keep rule was removed from kotlinx-coroutines-core (which is good), but
this caused AndroidDispatcherFactory to be minimized in newer R8 versions. Adding a rule explicitly
fixes the problem.

ui/kotlinx-coroutines-android/resources/META-INF/com.android.tools/r8-from-1.6.0/coroutines.pro

@@ -9,6 +9,8 @@
  boolean ANDROID_DETECTED return true;
 }
 
+-keep class kotlinx.coroutines.android.AndroidDispatcherFactory {*;}
+
 # Disable support for "Missing Main Dispatcher", since we always have Android main dispatcher
 -assumenosideeffects class kotlinx.coroutines.internal.MainDispatchersKt {
  boolean SUPPORT_MISSING return false;