disableSelf doesn't recover the init state of value

It leaves empty string instead of null for the value of
secure.enabled_accessibility_services. It may cause other tests
fails.

Bug: 111317486
Test: atest CtsAccessibilityServiceTestCases
Test: manual check the value of settings providers after test
Change-Id: I3f882ba00145ffa555514117c4684aec9ce33947
1 file changed