Make DPM/DPMS unit-testable

- Now all services that DPMS uses are injectable.
- Introduce some wrappers to make static methods and final class mockable.
(e.g. for Binder.getCallingUid())

- In unit tests we replace those with Mockito mocks, except we use a partial
mock for PackageManager, because we use way too many methods of this and
most of them are okay to use directly.

- To install a partial mock to PackageManager, I needed to make
ApplicationPackageManager @hide public non-final.

- For a starter, added tests for DPM.setAmin().

Bug 24061108

Change-Id: I2afd51d8bc0038992d5f9be38c686260be775b75