Fix homepage cards show up problem after first launch

ContextualCardLoader was waiting for eligible card checkers with a 300
ms timeout one by one in a loop. So if more than four checkers expire,
the total waiting time will exceed the 1 sec homepage content loading
timeout, which causes no card shows up.

The solution is to leverage all the checkers' timeout starting time,
and increase the timeout for all checkers. So that ContextualCardLoader
just needs to wait for all checkers for at most 400 ms and then can get
the results.

Bug: 159236069
Test: robotest
Change-Id: I601ac4151bf8be68b30eaabdb45a4e1ace95653f