Switch trust agents to extend unlock mode by default.

This CL changes the default behavior for trust agents to extend unlock
mode.

The Q droidfood phase allowed trust agent behavior to switch between
active unlock (legacy behavior) and extend unlock modes --- where the
extend unlock mode would prevent a trust agent (such as Smart Lock)
from actively unlocking a locked phone, instead only allowing it to
extend the duration that an already unlocked phone can continue to
stay in the unlocked state. In preparation for Q proper, the developer
options that controlled this toggle are being removed, and extend
unlock mode is the only allowed mode of behavior for trust agents.

Bug: 120871688
Test: Trust agents (like Smart Lock) only extend unlock on Q
Merged-In: Ifad45f90e0f2b5c3d180a74a97a89d45d75cb02a
Change-Id: Ifad45f90e0f2b5c3d180a74a97a89d45d75cb02a