Disable navbar contrast for ThemedNavBarKeyboard

With Ib8f4045b9e8e7bf65c13437133f114d91d0cb050 navbar always has
contrast thereby creating a solid navbar. However, for IMEs that wish to
draw a gradient, will need to have this disabled.
This change disables contrast for extended navbar gradient.

Fix: 130809016
Test: Manually verified:
    1. m -j ThemedNavBarKeyboard
    2. Install and enable
    3. Click "Extended dark nav bar" and verify gradient is visible.

Change-Id: I511bdcde0ef57c1484faace484db262b49fdfe16