Control AccessibilityServices with one setting.

Accessibility services were controlled by two settings:
ENABLED_ACCESSIBILITY_SERVICES and ACCESSIBILITY_ENABLED. These
settings needed to be changed together, and race conditions could
creating inconsistent states as ACCESSIBILITY_ENABLED was also
changed by the framework itself.

Restructuring to keep ACCESSIBILITY_ENABLED up-to-date, but not to
use it to decide whether or not to enable accessibility. Enabling
is controlled solely by ENABLED_ACCESSIBILITY_SERVICES.

Bug: 27455234
Change-Id: I9ab1e5936080c1b0dd8410dd6e8666e7f6eaccf3