Turn mLocalPolicies into a map

mLocalPolicies may be modified in some functions of policy engine, also
while being iterated over. As a first step of making this a bit more
safe, turn mLocalPolicies into a hash map instead of a sparse array.
This way, one can get a copy of all of the keys before iterating, hence
at least keeping the users that we iterate over stable during one loop.

Bug: 397370774
Test: btest a.d.c.DeviceManagementCoexistenceTest
Flag: EXEMPT refactor
Change-Id: I6f35375f3ddcbf2f9ab104e5db00e1e16a259123