StatusBar: add DISABLE2_GLOBAL_ACTIONS.

We add a new StatusBarManager flag to disable the global actions dialog
(a.k.a. the power button menu), which is required in certain kiosk mode
scenarios.

Bug: 65813398
Test: runtest systemui
Test: manual, by using a modified version of TestDPC and
      DevicePolicyManagerService

Change-Id: Ib7da2f4837281e98e558afcc30be8bae6f2e179a