Convert preview sdkVersion to int for target-api

If the target SDK of the module is a preview, the current logic passes
it through to llvm-rs-cc, which expects an int. Convert codenames to
their API level first, then extract the final or preview int.

Also simplify the logic to look for 'current' more generically via the
common ApiLevel library functions.

Test: Verified local build of riscv64, confirmed RS attempts to build
with API level 9000+ instead of 'VanillaIceCream'.

Change-Id: I1fa7577181bfd16de0ea4e77f7ab8fbd8fdb55e5