Move the alpha value to the correct bits

The alpha value should be at top 8 bits. The original implementation of
navigation bar color scrim missed 2 zeros of SCRIM_ALPHA, which made the
color always transparent.

Fix: 330257475
Bug: 309578419
Test: Open a hello-world app in 3-button nav mode. See if nav bar has
      enough contrast.
Change-Id: Ifab587bc1b92225c4dc6ee79558a2ce628b47304