Clear settings provider before testing in A11yShortcutControllerTest

Some tests in AccessibilityShortcutControllerTest are accessing to
settins, but static Settings.ContentProviderHolder may hold a real
provider before testing and cause failure with message "Failed to lookup
info for package null"

Bug: 136786077
Test: atest AccessibilityShortcutControllerTest
Change-Id: If691192e0e210b39d315248cfa087260fb20117a
1 file changed