Evaluation delay = 8 secs for explicitly selected networks

When a network was explicitly selected, the user just tapped it and
is likely waiting for it to connect. A shorter timeout is therefore
more appropriate for these.

Bug: 281970908
Test: New test in ConnectivityServiceTest
(cherry picked from https://android-review.googlesource.com/q/commit:0f1413395df672328c3e77cccc0ca3600b2bec84)
Requested by Samsung for their upcoming U release
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:4134c26ebf40b9e8dc38b1ec058e8018e93fc597)
Merged-In: I99a263960cbc9292868fc4bb46a4f3b2f3c71541
Change-Id: I99a263960cbc9292868fc4bb46a4f3b2f3c71541