audio policy: fix several device descriptor issues.

- checkOutputsForDevice() should only clear device descriptor attributes
if the device is digital.
- checkInputsForDevice() did not clear device descriptor attributes at all.
- AudioPort::clearCapabilities() and importAudioPort() should not
manage gains as these are device specific.
- importAudioPort() should load a default port config.

Bug: 21990937.
Change-Id: Ida762ed8f9baaabae392cb4291eff1a8d3009751