Make Navbar Back Button UI Consistent with SysUI

Change the back button (3-button nav) from a chevron during setup wizard into a triangle to be more consistent with the rest of SysUI.

Flag: NONE
Test: Manually tested in Pixel 7 setup wizard mode.
Bug: 293260092
Change-Id: I4dd5ff980f386ed108f12c7614c8f1217bc07e59
2 files changed