Launch CircularRevealActivity as singleInstance

Start the activity as the only activity in its task so that
Dialer will not remain on the task stack after the call is ended.
Bug: 18932141

Change-Id: I18f780df4a9ddfe2bef56f27b79a9ff5efbe9d86
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index e8def4b..039cc31 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -277,6 +277,7 @@
                   android:theme="@style/Theme.CircularRevealAnimation"
                   android:exported="false"
                   android:configChanges="keyboardHidden|orientation"
+                  android:launchMode="singleInstance"
                   android:noHistory="true"
                   android:excludeFromRecents="true"
                   android:screenOrientation="nosensor" />