System not checking if enabled services are installed when disabling accessibility.

The system is didabling accessiblity if no accessibility serivces are enabled
to avoid sending events across processes if no recipients are present. The
check considers enabled services which may not have been installed. Now the
check is made against enabled and installed serivces.

bug:5347273

Change-Id: Iad391a1a5bf0bbca470584bc8392f35821ba768c