Add freeze period support in SystemUpdatePolicy

Freeze period is defined as a pair of calendar dates (recurring annually)
during which the system should block any incoming system updates, including
security patches. They are set on top of existing system udpate policy
types (automatic, windowed, postpone) such that outside the freeze
periods existing policy semantics will still apply. They are created to
allow admin to keep their device fleet from any destabilizing changes during
critical period of the year, for example during Christmas sales period.

Device Owner can set several freeze periods, although to prevent the device
from not receiving OTAs indefinitely, each single freeze period is
restricted to be at most 90 days, and adjacent freeze periods need to be at
least 60 days apart. To properly enforce these restrictions, any freeze
periods the device previously experienced is tracked by DevicePolicyManager
and are validated against any new policy. This is to deal with corner cases
such as the admin repeatedly set a short but overlapping freeze period on a
rolling basis, hence bypassing the 90-day freeze period restriction.

Test: runtest -c com.android.server.devicepolicy.SystemUpdatePolicyTest frameworks-services
Bug: 64813061
Change-Id: I2864192797dc194edd9c183b881da6cfe3fdba5e