Fix CentralNotice Underallocation

CentralNotice should have been giving at least 1 slot to each banner.
In the case where low weight banners were at the end of the banner
list they could be missed during the reallocation step.

This has been solved with sorting, explicit minimum, and taking
care of any potential overallocation.

Change-Id: I2214ed49ede39718bc22cb69e9662ad344beacc6