hci: call into L2C for key requests for LE connections

Call L2CAP for LE keys. When a request comes in, L2C code will
be asked for a key and will have the option of saying "yes here
it is" or "no"

BUG=chromium:805112
TEST=build

Change-Id: I2fd66caef5127d07d1de27bf474547d251a9349d
4 files changed