UsbSerialProber: API cleanup; support multi-port devices.

acquire() is renamed and deprecated in favor of findFirstDevice().

findAllDevices() is added as new functionality.

Internally, probe() now returns a list, allow a (future) multi-port
device to return multiple drivers, one for each port (googlecode
issue #14).

UsbSerialExamples/src/com/hoho/android/usbserial/examples/DemoActivity.java

@@ -110,7 +110,7 @@ public class DemoActivity extends Activity {
     @Override
     protected void onResume() {
         super.onResume();
-        mSerialDevice = UsbSerialProber.acquire(mUsbManager);
+        mSerialDevice = UsbSerialProber.findFirstDevice(mUsbManager);
         Log.d(TAG, "Resumed, mSerialDevice=" + mSerialDevice);
         if (mSerialDevice == null) {
             mTitleTextView.setText("No serial device.");