btm: Add a way for CSIS to register scan callback

When registered, callback will be called whenever any scanning is ongoing.
It will be usefull for native service (CSIS) wanting to hook into scanning
performed by the Settings application. CSIS can also start or stop the
scan procedure with respect to the main scanner. It will not interrupt nor
change the scan duration if the main scanner is already running.

Bug: 150670922
Tag: #feature
Test: compile & manual testing
Sponsor: jpawlowski@
Change-Id: Ic0cc508ec5881d0c84dfd919ddb38618164ffbeb