Expose PermissionChecker as system API.

Only the most basic two APIs are exposed here, which happens to be the
ones required by USB module so that we can unblock them before IC. The
other overloads and variants can be exposed later when we have a
proper decision on how to expose this entire class as an API.

The constant values are hard-coded because they have to be compile
constants to be included in API, while referencing the PermissionChecker
fields doesn't count as such.

More details are available in Buganizer comments.

Bug: 195353742
Test: atest android.permission5.cts.PermissionCheckerTest
Change-Id: I156c3be0e4c45c95a65bfa9117fb6b850b95238d