Added Activity flags to show on lock screen.

Added two new flags to the Activity to turn the screen on and
show on the lock screen. These can be used instead of the Window flags
LayouParams.FLAG_TURN_SCREEN_ON and LayoutParams.FLAG_SHOW_WHEN_LOCKED
to prevent the double onStart/onResume lifecycle events.

The flags can be set as an attr for the Activity in the AndroidManifest
using android:showWhenLocked="true" and android:turnScreenOn="true".
They can also be set through methods in the Activity class using
setShowWhenLocked(true) and setTurnScreen(true).

Fixes: 36850100
Test: Created sample application, tests/ShowWhenLockedApp, that set the
      flags in the manifest and code. Tested multiple scenarios to
      launch the Activity with the flags set and unset.
Test: cts-tradefed run commandAndExit cts-dev --module CtsServicesHostTestCases -t android.server.cts.KeyguardTransitionTests
Test: cts-tradefed run commandAndExit cts-dev --module CtsServicesHostTestCases -t android.server.cts.ActivityManagerActivityVisibilityTests
Test: cts-tradefed run commandAndExit cts-dev --module CtsServicesHostTestCases -t android.server.cts.KeyguardTests

Change-Id: I44f0e313df4531d49c7ac56108b6bf80e41fefc1