AudioSystem: startAudioSource: ensure port handle initialized

In case of failure, port handle reported is not initialized
by JNI layer. Any failing call to native AudioSystem::startAudioSource
will not set the port handle, but returning an error status.
JNI checks for port handle validity rather than error status.

This CL ensures JNI checks first status to report an error, otherwise
asserts on handle validity.

Bug: 186088557
Test: make

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: I4eb8046a1263aeb7d464d35a360a8adb77c4b1a5