Fix undefined port id value in

AudioPolicyManagerTestMMapPlaybackRerouting

Although this is an output parameter, AudioPolicyManager::getOutputForAttr
verifies that initial value is AUDIO_PORT_HANDLE_NONE.
Because the field was not explicitly initialized in the test,
it contains undefined data and might cause tests to fail randomly.

Bug: 290129288
Test: atest audiopolicymanager_tests
Change-Id: I8662816c45d179305dcc27041dbb6b1d9afcc5e1