Make navigation bar color transparent by default

This CL only changes the default color from black to transparent. If an
app specifies the navigation color from the resource or programatically,
its color will still be applied.

This feature is only enabled when config_navBarDefaultTransparent is
enabled.

Fix: 232195501
Test: Open Pixel Logger or NetGrapher with the combinations of:
      1. gesture navigation or 3-button navigation
      2. light theme or dark theme
Change-Id: I9f1fd67e1c42cc732accba2260e0374cbc495041