Return the matched event from TestableNetworkOfferCallback

Since the requestId is generated by CS, the only way to get a hold of it
is by intercepting the onNetworkNeeded() callback.

Test: TH
Change-Id: I8d66d287633151f9cfe7053c2b934121d2c96dbc
diff --git a/staticlibs/testutils/devicetests/com/android/testutils/TestableNetworkOfferCallback.kt b/staticlibs/testutils/devicetests/com/android/testutils/TestableNetworkOfferCallback.kt
index 21bd60c..a0078d2 100644
--- a/staticlibs/testutils/devicetests/com/android/testutils/TestableNetworkOfferCallback.kt
+++ b/staticlibs/testutils/devicetests/com/android/testutils/TestableNetworkOfferCallback.kt
@@ -52,10 +52,11 @@
 
     inline fun <reified T : CallbackEntry> expectCallbackThat(
         crossinline predicate: (T) -> Boolean
-    ) {
+    ): T {
         val event = history.poll(timeoutMs)
                 ?: fail("Did not receive callback after ${timeoutMs}ms")
         if (event !is T || !predicate(event)) fail("Received unexpected callback $event")
+        return event
     }
 
     fun expectOnNetworkNeeded(capabilities: NetworkCapabilities) =