No longer unlock activity manager to test whitelist

In order to check the DevicePolicyManagerService locktask whitelist
the activity manager had to release its lock preserving internal
state. That is undesirable and not scalable now that we need to check
the whitelist at startup for bug 19995702.

This change causes DPMS to update activity manager with the whitelist
whenever it changes so that activity manager can check the whitelist
without releasing the acitivty manager lock.

Change-Id: I3ed6eb5ceae2cd7e7ae3280abd708d5ce43a2851