Add a field and score flag for first evaluation

A wifi network that is still evaluating, i.e. that doesn't know
yet if it's behind a captive portal, should not be preferred
to a network that yields to bad wifi because the stack doesn't
know yet if it's bad.

To rank the networks correctly, the ranker will therefore need
to know whether a network is still being evaluated. This patch
adds the time when this happened first to the NAI (as a
timestamp for debuggability) and the corresponding flag in
FullScore.

This doesn't have new tests because it doesn't yet expose
new behavior. Tests will come with the behavior.

Test: FrameworksNetTests
Change-Id: I737f314760356926fc07e6eef52f3c8abba2248b