Register for Service Changed notification after encryption

This fixes issue introduced in commit ddf6320b
Registration for Service Changed notifications was
disabled, because there was no good way back then to have
"opportunistic" app. Since opportunistic apps were introduced, and the
code in GATT_ConfigServiceChangeCCC is already marking connection as
opportunistic, reenable notification registration.

Test: manual verification, that Service Changed descriptor is set
Change-Id: Iefaec3914829fa4a4a60dc3c3790c3daa9f4f4d8