Fix getprop Format

getprop used to output a colon after the property name like:
[property.name]: [property.value]

Add back the colon that was missing, because
com.android.ddmlib.GetPropReceiver's regex expects it. Without
the colon, the GetPropReceiver doesn't parse the device's
properties causing CTS to not recognize the device.

Change-Id: I9bef5ab2b310c831c49c8c51cae7f129167c2dc5