Check target is empty before showing it in AccessibilityFloatingMenu

Root Cause: From ag/17686873, we remove the protect mechanism that
AccessibilityManagerService might not ready to retrieve information when
the phone get booted without keyguard lock, but it will still happen if
the sequence of the object created got change, just like b/184272107 in android sc-dev.

Solution: Change to check target is empty first before showing it, this
way, we can show a11y FAB both in onUserUnlocked() and
onKeyguardVisibilityChanged(false) stage without showing empty content
a11y FAB.

Bug: 192635136
Fix: 184272107
Test: atest AccessibilityFloatingMenuTest
Change-Id: Icca1c2bbae3f0bf6cad4fb28c53b5a9e6cd9b271