Limit the number of routes for performance

In evaluating whether "most" of the addressing space is
covered, the list of routes are obtained from a third-party
app, so it's possbile the system service stalls unless
some limit is enforced on how much work it has to do.
This change limits the number of routes to 400, as determined
by time measurement on various devices.

Bug: 74176086
Test: runtest framework-net
Change-Id: Ie4a96098bc044ade87b188839586f14dd101c100