Permission Check For DPM.getPermittedAccessibilityServices

Require the callers of DPM.getPermittedAccessibilityServices(userId) to
hold the MANAGE_USERS permission.  The only known callers of this API
are settings apps that already hold this permission.

Bug: 62343414
Test: com.android.server.devicepolicy.DevicePolicyManagerTest
Test: com.google.android.gts.devicepolicy.DeviceOwnerTest
Test: Manually checked accessibility settings in DO and PO modes
Change-Id: I8ee3f876fcaffa63636645f0f59709cd147254ef