Rename HotspotListener to BssidListener

And other usage of term Hotspot. This is to be technically correct
with the name usage, should the API become public later.

This change also fixes few outstanding requests from GScan API.These
include

a. An event with scan results when REPORT_AFTER_EACH_SCAN is set.
b. ScanSettings now takes in number of APs as an argument.
c. Removed single shot scan API (since it is not implemented anyway)

Change-Id: Id160f9f14193a37cce5f3ab8d6ea528344b788a0