Synchronize managed services approval list

Some requests to read/write the list come in on other threads and
could cause system process to crash (esp when in the process of saving
the policy file)

Test: reboot, use device, change approval list in settings. take bugreport
Fixes: 132976941
Change-Id: I282d1e763079da1fee2c96266be39e6ef3b131ed
(cherry picked from commit b776e397)