Use android-support @keep annotation to avoid removal of AndroidExceptionPreHandler

Fixes #214 better

Author: elizarov

README.md

@@ -107,9 +107,3 @@ To avoid field overloading by type during obfuscation, add this to your config:
     volatile <fields>;
 }
 ```
-
-You also need to keep this class if you build your Android releases with `minifyEnabled true`:
-
-```
--keep class kotlinx.coroutines.experimental.android.AndroidExceptionPreHandler
-```

ui/kotlinx-coroutines-android/build.gradle

@@ -1,5 +1,10 @@
+repositories {
+    google()
+}
+
 dependencies {
     compileOnly 'com.google.android:android:4.1.1.4'
+    compileOnly 'com.android.support:support-annotations:26.1.0'
 }
 
 tasks.withType(dokka.getClass()) {

ui/kotlinx-coroutines-android/src/main/kotlin/kotlinx/coroutines/experimental/android/AndroidExceptionPreHandler.kt

@@ -1,9 +1,9 @@
 package kotlinx.coroutines.experimental.android
 
-import kotlinx.coroutines.experimental.CoroutineExceptionHandler
-import java.lang.reflect.Modifier
-import kotlin.coroutines.experimental.AbstractCoroutineContextElement
-import kotlin.coroutines.experimental.CoroutineContext
+import android.support.annotation.*
+import kotlinx.coroutines.experimental.*
+import java.lang.reflect.*
+import kotlin.coroutines.experimental.*
 
 private val getter =
     try {
@@ -20,6 +20,7 @@ private val getter =
  *
  * @suppress This is an internal impl class.
  */
+@Keep
 class AndroidExceptionPreHandler :
     AbstractCoroutineContextElement(CoroutineExceptionHandler), CoroutineExceptionHandler
 {