Only send messages to CHRE if client is registered

In the previous API design, we only allowed the ContextHubClient
to be used if a ContextHubClientCallback was supplied. Since we are
updating the API to allow a callback xor a PendingIntent, change
sendMessageToNanoApp to reflect this.

Bug: 117612105
Test: Compile only
Change-Id: Ic1ef7c77b95699a00fe29c5941a53af37e9385de