Add protection against LE scanning abuse

Added two checks to prevent abuse. The first check ensures that an
app doesn't scan too frequently in a certain time period. It is
allowed to scan again after its oldest scan exceedes said time
period. The second check ensures that an app doesn't scan for too
long. Upon starting a scan, this code waits a certain amount of time.
If the app is still scanning by that point, this code stops
the scan and forces the app to use opportunistic scanning instead.

Bug: 27357274
Change-Id: Ic99ac1f838e15ed99fe2fae643ef073d74b5c96b