Remove Guava references from Telecom.

Prepping telecom to run in system-server process by removing references
to guava.

Bug: 18112269
Change-Id: I994be480284462240da46c9cf0e47aa83f5f1fe9
diff --git a/src/com/android/server/telecom/CallsManager.java b/src/com/android/server/telecom/CallsManager.java
index 5c8f24f..eb5b7ab 100644
--- a/src/com/android/server/telecom/CallsManager.java
+++ b/src/com/android/server/telecom/CallsManager.java
@@ -40,9 +40,7 @@
 
 import com.android.internal.util.IndentingPrintWriter;
 
-import com.google.common.collect.ImmutableCollection;
-import com.google.common.collect.ImmutableList;
-
+import java.util.Collection;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
@@ -314,8 +312,8 @@
         return true;
     }
 
-    ImmutableCollection<Call> getCalls() {
-        return ImmutableList.copyOf(mCalls);
+    Collection<Call> getCalls() {
+        return Collections.unmodifiableCollection(mCalls);
     }
 
     Call getForegroundCall() {