init: Fix sync issue on property trigger.

This issue reproduces in the following senario.
 1. ("", "") is added to queue;
 2. property_triggers_enabled is set to 1;
 3. user defined property is triggered, like sys.usb.config=adb;
 4. ("sys.usb.config", "adb") is added to queue;
 5. main loop interpret ("", "") and queue all current triggers to execution,
    so ("sys.usb.config", "adb") is queued for execution for the first time.
 6. main loop interpret ("sys.usb.config", "adb"), it is queued for
    execution for a second time.
The second time makes ASIT fail.

Bug: http://b/28218187


Change-Id: I230e175e0dca8989f1e5bd812398da90082d0ec1
Signed-off-by: caozhiyuan <cao.zhiyuan@zte.com.cn>