Hide a11y usage when there is no enabled a11y service

A11y usage still shows in privacy page after
user disable a11y with a11y usage dialog.

We need to check again whether there is any enabled a11y
after screen comes back to foreground.

Change-Id: Icf7aa7dc9d802765ec49df8651385397f454cd48
Fix: 140207216
Test: robotest