Add prerelease driver to graphics driver preference dashboard.

To facilitate better development, add prerelease driver choice to graphics
driver preferences dashboard. This patch also makes sure that when game driver
or developer driver is not shipped, the choice doesn't show up.

Minor: Add tests.

Bug: b/148626177
Test: make RunSettingsRoboTests ROBOTEST_FILTER=GraphicsDriver
Change-Id: Iad398cd663300086d3f4802ffab34531e11dfb17