Cleanup some more DPM tests to use runAsCaller.

Factored MockSystemServices out of DpmMockContext. It contains all system
mocks that should be shared by all contexts. So now we can have several
contexts in the test without having to adjust mocks behavior for each of
them separately.

+ minor cleanup: imports, lambdas, redundant generic arguments,
  unnecessary try-finally

Bug: 38445735
Test: runtest -x frameworks/base/services/tests/servicestests/src/com/android/server/devicepolicy/DevicePolicyManagerTest.java
Change-Id: Id18b574b021c16f86fda8ae31291dd5defec0004