use a String for IM_PROTOCOL

fix exception when putting jid to contacts:

W/Bundle  (19166): Attempt to cast generated internal exception:
W/Bundle  (19166): java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
W/Bundle  (19166): 	at android.os.Bundle.getString(Bundle.java:1121)
W/Bundle  (19166): 	at com.android.contacts.common.model.RawContactModifier.fixupLegacyImType