Factor out NetworkID and caching mechanism from n_q_e.{h,cc}

This keeps the caching mechanism separate from NQE. This
is first step towards enabling persistent caching of
estimates.

This CL does not record any functional changes, although
tests have been expanded.

Also, add UMA to record how frequently cached network
quality is available.

BUG=490870

Review-Url: https://codereview.chromium.org/2128793003
Cr-Commit-Position: refs/heads/master@{#407521}
14 files changed