Add missing Gradle wrapper

android/gradle/wrapper only held the properties file, so the project
could not be built from the command line at all. Generate the wrapper
for the pinned 8.11.1 distribution and ignore the .kotlin scratch dir.

android/.gitignore

@@ -16,3 +16,6 @@
 keystore.properties
 *.jks
 *.keystore
+
+# Gradle build scratch
+.kotlin/