Fix test DevicePolicyManagerTest#testSetRequiredStrongAuthTimeout_DeviceOwner

1) Started returning the default value for getLong() on SystemProperties mock
2) Added a test that the minimum timeout cannot be changed using a system
   property on non-debuggable builds
3) Added new within range test for completeness.
4) Started using TimeUnit instead of ms constants.

Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/devicepolicy/DevicePolicyManagerTest.java
Bug: 34317979
Change-Id: I0409451ae39e74ec3d96a098042302291ec3408f