Fix regressions in flushRoutes().

http://ag/486276 left out a comma, causing the command to be "routeflush" (which
is invalid), instead of the correct "route flush".

http://ag/495068 changed the order, causing the interface to be removed from the
interfaceToIndex map before we are done using it in modifyPerNetworkRules().

Change-Id: I65af0b5763b13e47b48e7e2b81d243dc19e1a03b