Fix incorrect pos / id for privacy dot

When only left / right overlays are enabled because of long edge cutout,
we shall assign layout id to correct local variable, and do some
adjustment of layout gravity, and padding.

Bug: 201481944
Bug: 203361384
Test: atest com.android.systemui.ScreenDecorationsTest
Test: manually rotate, enable RTL, disable round corner, long side
      cutout, and display size change, to make sure privacy dot is shown
      on correct pos

Change-Id: Ife8f81e33f44e11cf2be40f74669ee9a9b31f707