Fix missing showWhenLocked activity attribute

This attribute will stabilize ScreenDecorWindowTests since this test
needs activity is shown even on locked device.

Fixes: 119041502
Bug: 119037340
Test: On sleeping DUT.
  atest FrameworksServicesTests:com.android.server.wm.ScreenDecorWindowTests
Change-Id: I98ef00ba32c792c396f3abba447dd20628902492
Merged-In: I8fc2683ad65e627c4e002b767fd23a4dd9f16223