Respect KEYGUARD_DISABLE_SHORTCUTS_ALL

When DevicePolicyManager#getKeyguardDisabledFeatures includes a set bit
for the KEYGUARD_DISABLE_SHORTCUTS_ALL flag, we disable lock screen
shortcuts.

Fix: 268218507
Flag: customizable_lock_screen_quick_affordances
Test: faked the returned value and manually verified that the feature is
gone from settings, wallpaper picker, and the lock screen
Test: unit test case added

Change-Id: I6c8a2d1fd2f891c9cd483d395c8175d6e8361b1b