Apply API review comments for SHOW_WITH_INSECURE_KEYGUARD

Applied review comments but keeping the flag hidden for now.

Bug: 36555156
Test: android.server.cts.ActivityManagerDisplayTests
Test: #testExternalDisplayActivityTurnPrimaryOff
Test: #testLaunchExternalDisplayActivityWhilePrimaryOff
Test: #testShowWhenLockedPublicVirtualDisplay
Change-Id: I1aa1289424d0d778bc0668c9ec69bc56f47891e3