We already have outer_width and outer_height in wear UI, and x_offset
and y_offset in VR UI. This CL adds margin_width_ and margin_height_ to
their base class (ScreenRecoveryUI) to shorten the gap. This will be in
general useful for round or round-cornered screens.

Move the density computation to ScreenRecoveryUI ctor so that the value
can be used earlier.

Bug: 62732748
Test: Setting and not setting margin_{width,height}_ on angler. Check the
      recovery texts (recovery menu as well as 'View recovery logs').
Change-Id: Ibf6238c9cc8949a42ed8a410e1c09d55b0b5a151
(cherry picked from commit 87f46508)