Improve test coverage for FakeValueGenerator.

Bug: None
Test: atest FakeVehicleHalValueGeneratorsTest
Change-Id: I6df508e148ff0348fc28b467b4e5a70cf5727a27
Merged-In: I6df508e148ff0348fc28b467b4e5a70cf5727a27
(cherry picked from commit 6ae468c1e39f72456416697ee7cf483dcf0c6f3b)
4 files changed