stingray: ril: Use /dev/usb/tty* symlinks instead of /dev/ttyUSB?

/dev/ttyUSB? files are created depending on the order USB devices are
probed. Make the ril use /dev/usb/tty* symlinks instead.

Change-Id: Ie0c66aead69e7c76dd592bfc0f28594c5f423956
Signed-off-by: Benoit Goby <benoit@android.com>

system.prop

@@ -1,14 +1,14 @@
 # RIL and telephony related settings
 rild.libargs=-d /dev/chnlat10
 rild.libpath=/system/lib/libmoto_ril.so
-persist.ril.mux.ttydevice=/dev/ttyUSB2
+persist.ril.mux.ttydevice=/dev/usb/tty2-1:1.2
 persist.ril.mux.noofchannels=8
 persist.ril.modem.mode=1
 ro.cdma.home.operator.numeric=31020
 ro.cdma.home.operator.alpha=Verizon
 ro.cdma.homesystem=64,65,76,77,78,79,80,81,82,83
 ro.cdma.data_retry_config=default_randomization=2000,0,0,120000,180000,540000,960000
-persist.ril.modem.ttydevice=/dev/ttyUSB4
+persist.ril.modem.ttydevice=/dev/usb/tty2-1:1.4
 ro.telephony.default_network=4
 
 # The OpenGL ES API level that is natively supported by this device.