Add "throw" and "unreachable" routes to NetdBinderTest

Add "throw" and "unreachable" routes data to
NetdBinderTest#NetworkAddRemoveRouteUserPermission test case.

Modify 'ipRouteString' util method to correctly generate "throw" and
"unreachable" routes representation.

Bug: 186082280
Test: atest NetdBinderTest
Change-Id: Idd57eb85ffe2488e9ff6f25d68e58ef083c9cd6c
3 files changed