[PASSPOINT] Add metrics to determine deployment/avail of Passpoint

Add metrics which will help determine Passpoint availability in the
wild. The metrics measure the percentage of time a Passpoint network
is visible to user - by collecting historgrams of observations per
scan result.

Bug: 66951771
Test: unit test
Change-Id: I8c91f490a533f90a4f0630e1cb461e1d2643358b