Expose PackageState#getHiddenApiEnforcementPolicy

Adds PackageState method for @hide API enforcement policy, hidden
inside the state object so that the caller doesn't need to figure
out the effective state.

Although it's worth noting this isn't final state, since
ProcessList can use a device configuration to override policy.

Bug: 236389629

Change-Id: If9edd89186534d02933dac5f9e5ff063c27c52b6