Different hidden API enforcement policy for P & pre-P.

Also make these configurable so we have the flexibility to change it if
necessary.

Setting the policy inside ActivityManagerService is not ideal, as that
means that AMS is the only place where the policy in ApplicationInfo is
correct. It should really be set inside PackageManagerService. However,
if it's set there, it would get out of date when the settings change, and
we'd have to update inside AMS anyway. So putting it only here seems ok
for now.

Test: $ adb shell settings put global hidden_api_policy_pre_p_apps 2
Test: $ adb shell settings put global hidden_api_policy_p_apps 2
Bug: 64382372
Change-Id: Ic4cbbb1e6464623e90c17ae08c0b6cbbe0dfa125