Reset LE encryption key size at LE disconnection only

This change checks the transport type on receiving the disconnection
and resets the encryption key size only if the LE disconnection
has taken place.
This fixes the issue where read request to characteristics that
require encryption,  fails after cross key derivation due to
disconnection of BR/EDR transport.

Bug: 22515016
Change-Id: If6aad91a628eabbb5a4b7f5c22812fe94d4c5db2