call: show activity on incoming call and lock screen

* launch CallActivity on incoming call
* make CallActivity available on lock screen

Uses API 27 (Android 8.1) so update the platform.

Change-Id: Ie4f89fdd728def5c58fb1012b41ff141c22a3544
diff --git a/ring-android/app/build.gradle b/ring-android/app/build.gradle
index b1ef1c5..b7ebc7a 100644
--- a/ring-android/app/build.gradle
+++ b/ring-android/app/build.gradle
@@ -41,12 +41,12 @@
 }
 
 android {
-    compileSdkVersion 26
-    buildToolsVersion "26.0.2"
+    compileSdkVersion 27
+    buildToolsVersion "26.0.3"
 
     defaultConfig {
         minSdkVersion 18
-        targetSdkVersion 26
+        targetSdkVersion 27
         vectorDrawables.useSupportLibrary = true
     }