Don't type variables as booleans in mk2rbc

Our boolean support is not fully fleshed out enough to make variables
real booleans. Keep them strings for now to resolve CI failures.

Bug: 275865081
Test: ./build/bazel/ci/rbc_dashboard.py --quick with aosp/2526609
Change-Id: I636bd9f39b50a47ecf92aecd2bf8ea41eac4d604