Tweak SystemUpdatePolicy APIs

* Introduce a FreezePeriod class that represents a single freeze period
  and is defined by two MonthDay instances.
* Add ERROR_UNKNOWN to ValidationFailedException
* Make SystemUpdatePolicy final
* Document SystemUpdatePolicy.InstallationOption and add IntDef to getType()

Test: runtest frameworks-services -c com.android.server.devicepolicy.SystemUpdatePolicyTest
Test: cts-tradefed run cts-dev -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.DeviceOwnerTest#testSystemUpdatePolicy
Test: gts-tradefed run gts-dev --module GtsGmscoreHostTestCases --test com.google.android.gts.devicepolicy.DeviceOwnerTest#testSystemUpdatePolicy
Bug: 74976911
Change-Id: I85cf636c3a98c97bd03b7b296c3130028051a791