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/src/main/AndroidManifest.xml b/ring-android/app/src/main/AndroidManifest.xml
index 7cbba2d..0dfe1a0 100644
--- a/ring-android/app/src/main/AndroidManifest.xml
+++ b/ring-android/app/src/main/AndroidManifest.xml
@@ -157,6 +157,7 @@
             android:label="@string/app_name"
             android:screenOrientation="fullUser"
             android:theme="@style/AppTheme.ActionBar.Transparent"
+            android:showOnLockScreen="true"
             android:windowSoftInputMode="adjustPan|stateHidden">
             <intent-filter>
                 <action android:name="android.intent.action.CALL" />
@@ -276,6 +277,7 @@
 
         <activity
             android:name="cx.ring.tv.call.TVCallActivity"
+            android:showOnLockScreen="true"
             android:theme="@style/AppThemeBase">
             <intent-filter>
                 <action android:name="cx.ring.action.CALL" />