Start client only if requested when ensuring activity visibility

If the caller specifies notifyClients=false, it should only update
server side states without affect the client.

A corner case:
realStartActivityLocked for an attaching non-top visible activity
 beginDeferResume
  ensureVisibilityAndConfig
   > ensureActivitiesVisible(notifyClients=false)
    > realStartActivity the real top (attached but not finishAttached)
     > Unable to be resumed becase readyToResume() is false
       by the beginDeferResume
 endDeferResume

Bug: 352423986
Flag: EXEMPT bugfix
Test: atest TaskTests#testEnsureActivitiesVisible
Change-Id: I16b0e7facd8cfe6d6ed2f6eeb9da78835afbf471