Show shapes for nav buttons in debug view

At the moment we show BACK, HOME and RECENT_APPS when using the
navigation buttons; those buttons have geometric shapes (triangle,
circle and square) assigned to them that have a Unicode
representation that we can use instead, similar to other characters
like arrows and Enter.

Bug: 326242357
Test: Flashed on device
Change-Id: I37883b6ba67118781ee9e6edb97d2f1ac5e93f89