DO NOT MERGE NuCachedSource2: fix possible erroneous early free

Because the constructor of NuCachedSource2 sent a message to
AHandlerReflector object, AHandlerReflector::onMessageReceived could
have executed just before the object gets wrapped in a strong
pointer, resulting in erroneous early free. Fix the issue by using
static Create function to ensure the message is sent after the
object is wrapped in a sp.

Bug: 23882800
Change-Id: I38a9d7a3083f184b4c81d0b00ba1661721278855