Implement ShowMenu for StubRecoveryUI

The ShowMenu in StubRecoveryUI used to return initial selection
immediately. This leads to an immediate reboot if we enter recovery
with a stub UI and wait for user's input. This cl changes the behavior
of stub UI's ShowMenu so that it waits keys in loop, and only returns in
the case of interruption or time out.

Bug: 130535924
Test: start recovery with a stub UI, wait for the timeout
Change-Id: I96ad8aad8930756b09101bb98ba5fbe7b53bcb37