Fix #296, prepare was running plugin add each time

The problem was due to case mismatch. In cordova plugin ls a plugin shows as
org.chromium.polyfill.CustomEvent (note the capital C and E)
while in list of default plugins in cca it's listed as
org.chromium.polyfill.customevent.

Cordova plugin registry wants all ID's to be lower case. But allows upper case
letter in config.xml.