Allow connection parameters updates by slave

GATT service discovery in the slave role does not update to fast connection
parameters when connected. Accordin to the 4.0 spec, the connection
parameter disable/enable call is used for only by the masster role.
The 4.1 and later spec allows the slave role to control the connection
parameter update so that a slave role GATT client can perform the service
discovery using fast connection parameters.

Also switched to requestiong 7.5ms connection interval by default during
service discovery.

Bug: 18266904
Change-Id: Ie6300587306be3d694937e3888069e252592be42