Don't expose default strong auth timeout as constant

The admin can instead use the value of 0 to reset to default.

Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/devicepolicy/DevicePolicyManagerTest.java

Bug: 31430135
Change-Id: I0d6b29ca4eca65d7ca72a8975a0c28c9050a946c