DO NOT MERGE Change pairing_cb to assume temporary pairing by default

When pairing takes place, the pairing_cb.is_temp flag indicates whether
a pairing is temporary or permanent. Link keys are not stored for
temporary pairings. Since this is a "positive" flag, resetting the
pairing_cb control block (ex. memset to 0), it will assume persistent
pairing by default. Under certain circumstances, this can lead to a link
key being stored for temporarily secured connection.

This patch reverses the flag to be a "negative" flag. Renamed to
"persistent_bond", the default 0 meaning is now used to indicate a
temporary bond. If the lag is not properly set now, it will default to a
temporary bond and will not save the link key erronously.

Bug: 18345373
Change-Id: I8558abc9eace609b1ba02485b0ff775fb6f3c6af