NativeDaemonConnector: add waitForCallbacks method

Add a method that allows callers to wait until all unsolicited
responses received from the native daemon during a command are
processed.

When commands are issued to a native daemon (such as netd) through the
NativeDaemonConnector we block until the command response is received.
Any responses or events that are a side-effect (considered
"unsolicited") of the command are placed in a Message and handled as
callbacks. The order of their processing is not guaranteed and, as we
have seen from bugreports, can be handled several seconds
later - causing the SoftAP that was just set up to be torn down
because a late interface down/removed is indistinguishable from a
new interface down/removed.

This CL adds a method that first checks to make sure callback thread
is not the same thread as used for the blocking call.  The new
waitForCallbacks method uses a CountDownLatch to force the calling
thread to wait until all unsolicited responses received from the
native daemon during the execution of the command are handled.

The wifiFirmwareReload method is also updated to use the new
waitForCallbacks method.

BUG: 27857665
Change-Id: I3e22978f720b1cbf57fbb64ad4fea73f8c2d408a