floss: Synchronize readline with foreground action

While ForegroundAction::Readline is happening, the readline task should
wait until it's permitted to do readline again. Otherwise when the
program quits, the readline task will be interrupted before it cleans up
terminal resources causing terminal hang.

Bug: 199213563
Tag: #floss
Test: Manual

Change-Id: I87afde2be3445c381c8a61cf0ab56c87144b5d39