Enforce that a PendingIntent has an explicit mutability

flag on creation.

This was previously a log.e, this change enforces this requirement.
See go/immutable-pendingintents for more context.

Bug: 160794467
Test: atest PendingIntentTest
Change-Id: If49be614125d7ac7eb556559ff32016f41beaef8

core/java/android/app/PendingIntent.java

@@ -352,7 +352,9 @@ public final class PendingIntent implements Parcelable {
 
         if (Compatibility.isChangeEnabled(PENDING_INTENT_EXPLICIT_MUTABILITY_REQUIRED)
                 && !flagImmutableSet && !flagMutableSet) {
-            Log.e(TAG, msg);
+            Log.wtf(TAG, msg);
+            throw new IllegalArgumentException(
+                "Please specify an explicit mutability flag (FLAG_IMMUTABLE or FLAG_MUTABLE)");
         }
     }