Allow adding EmbeddedWindows without host windows

In some cases privileged clients want to add embedded windows without requiring a host
window. Client with INTERNAL_SYSTEM_WINDOW can add such a window. The system
will still blame the calling PID if the embedded process stops responding but will ignore
pointerDownOutsideFocus events to this embedded window since it has no host.

Fixes: 144250438
Test: atest CtsWindowManagerDeviceTestCases:WindowlessWmTests
Test:
1. cherry-pick ag/9618804
2. adb shell content insert --uri content://settings/secure --bind name:s:window_magnification --bind value:s:1

Change-Id: Iceef70020c6ecd31b10e92a535c27458505c9ef0