sm: Add interface to start encryption

Clients call smStartEncryption to start encryption on a connection with
a device which has been previously paired and bonded before.

BUG=chromium:805112
TEST=Tested with newblued calling this function after gattClientConnect.

Change-Id: I3b04b0259fd0bf15c90bec92a3ab4aa33a98f4f3
2 files changed