Allow DevicePolicy caches to cache nulls

DevicePolicyManager caches are in front of binder APIs that may return
legitimate nulls.  This change allows those nulls to be cached.

Flag: EXEMPT bug-fix
Bug: 378762206
Test: atest
 * atest FrameworksServicesTests:DevicePolicyConstantsTest
 * atest FrameworksServicesTests:DevicePolicyEventLoggerTest
 * atest FrameworksServicesTests:DevicePolicyManagerServiceMigrationTest
 * atest FrameworksServicesTests:DevicePolicyManagerTest
 * atest FrameworksServicesTests:EnterpriseSpecificIdCalculatorTest
 * atest FrameworksServicesTests:OverlayPackagesProviderTest
 * atest FrameworksServicesTests:OwnersTest
 * atest FrameworksServicesTests:PolicyVersionUpgraderTest
 * atest FrameworksServicesTests:SecurityEventTest
 * atest FrameworksServicesTests:SystemUpdatePolicyTest
 * atest FrameworksServicesTests:TransferOwnershipMetadataManagerTest
 * atest MixedDeviceOwnerTest#testIsDeviceOrganizationOwnedWithManagedProfile
 * atest MixedManagedProfileOwnerTest#testIsDeviceOrganizationOwnedWithManagedProfile
 * atest MixedManagedProfileOwnerTest#testNetworkLoggingDelegate
 * atest OrgOwnedProfileOwnerTest#testIsDeviceOrganizationOwnedWithManagedProfile
 * atest OrgOwnedProfileOwnerTest#testNetworkLoggingDelegate
 * atest com.android.cts.devicepolicy.DeviceOwnerTest#testAdminActionBookkeeping
Change-Id: Ibce0ccc8191e9aff2b87a7967c5f47a8b7788e49