Update HandleMenu coordinates and tests

Updates HandleMenu position calculation so that:
1) AdditionalSystemViewContainer can take a global x,y position, whereas
   before it needed pre-adjusted coordinates to adjust to default
   Gravity.CENTER gravity of the LayoutParams. By changing the params'
   gravity to TOP-LEFT, callers can send global (x,y) position without
   additional adjustments.
2) Removed marginStart from x value of the HandleMenu position when in
   fullscreen/split - as that is only intended for the freeform's menu
   (to provide separation from the task edge). In fullscreen and split
   this cause the menu to be slightly off-center.
3) Updated HandleMenuTests so:
  - As a result of (1), expected position values use coordinates relative
  to the display bounds, which is consistent with the test comments. So
  there won't be expected negative values.
  - Added resource overrides for width/height/margins, so that changing
    the real resources don't cause the test to fail, nor changing the
    device's density, which would resolve the real values (in dp) to
    different than expected px.
  - Updated flagged tests to use @EnableFlags, otherwise flagged tests
    may not run in presubmit if the flag is disabled and are prone to
    breaking.

Bug: 349464246
Test: atest HandleMenuTests
Flag: com.android.window.flags.enable_additional_windows_above_status_bar
Change-Id: Iffde19fa49cf0db097b9dd4abf504289f2af823d