ThemeHostTest: support both non-finalized and finalized VIC

generate_images.py assumes cts/hostsidetests/theme/assets/$x exists,
where x is ro.build.version.codename (if not "REL") or
ro.build.version.sdk (if codename is "REL").

Commit c17682d01da moved cts/hostsidetests/theme/assets/VanillaIceCream
to cts/hostsidetests/theme/assets/35. This is correct for the `next`
release config, which sets the codename to REL, but breaks ThemeHostTest
on trunk release configs.

Create a symlink VanillaIceCream -> 35 to keep ThemeHostTest passing on
trunk release configs. When no release configs use `VanillaIceCream` as
codename, the symlink should be removed.

Bug: 335304236
Bug: 341095938
Test: atest 'CtsThemeHostTestCases:android.theme.cts.ThemeHostTest#testThemes' # on one of the trunk release configs
Change-Id: I8ebbc936e50f38204a5f46be7b3387f8ca84fdae