LE: Do not remove bond on encryption failure

If the encryption fails during the re-connection process with a
previously paired LE device, the pairing is removed. Thus, a single
connection failure could lead to the device be unpaired.

This patch prevents unpairing when encryption fails.

Additionally, the IRK is restored from NV ram to fix reconnection to
dual-mode devices.

Bug: 18072300
Change-Id: I077a16d0715fdaa07b95b1c664c18bd839754896