Fix issues with CustomerSession listeners (#856)

* Fix issues with CustomerSession listeners

**Motivation**
The previous implementation of `CustomerSession` listeners
had a reference to a single `CustomerRetrievalListener` and
`SourceRetrievalListener`. After a listener is called, it
would be nulled out.

This logic was buggy because it resulted in possible race
conditions in which two calls are both in flight, and the
first call that completes nulls out the listener for the
second.

**Summary**
The solution to this issue is to associate an `operationId`
(i.e. UUID) with each listener. This avoids the race condition
and ambiguity about which listener is being referenced.

Additionally, the listeners were being held with a strong
reference, which could lead to memory leaks if the listener
held a reference to an Activity.

**Testing**
Manually verified with samplestore and example apps.

stripe/src/main/java/com/stripe/android/AbstractEphemeralKey.java

@@ -205,7 +205,7 @@ String getType() {
         return mType;
     }
 
-    @Nullable
+    @NonNull
     protected static <TEphemeralKey extends AbstractEphemeralKey> TEphemeralKey
     fromString(@Nullable String rawJson, Class ephemeralKeyClass) throws JSONException {
         if (rawJson == null) {