Use constants=BuildConfig.class for robolectric

In robolectric tests, use @Config(constants=BuildConfig.class), which
is the "gradle way" to configure Robolectric. Added the hacks that
are needed to make that happen in our setup, like
PatchedGradleManifestFactory and the custom build directory lookup.

Having this change allows Robolectric tests to run against merged
resources, which means different build variants will now resolve
resources correctly. Previously a test would fail in, say, resolving
@style/SuwThemeGlif.

Also migrated GlifDimensionTest to run on Robolectric.

Test: ./gradlew test
Change-Id: Ia7231f95818891a649678a0a6da0220a44031bf7