Draw legacy nav bar background directly

If we only use a view for it, it may not be visible depending on
what the app will do with the DecorView, i.e. set a weird clip
region. To fix that, we also draw the legacy navigation bar
background separately in a post-draw callback.

Test: TikTok
Test: Material app is still able to set bar color
Fixes: 131211797
Change-Id: Id744b98780f8f7f6dcd3482803a680a8f93be4c5