[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: I51e05e9a075259544008107810c4489e8111cfcd
2 files changed