Emit errors on mixed property syntax

This commit refines `compat` condition in `parseProperty()` so that a
module definition would either use the new syntax or the old syntax,
but not something in the between, such as

    cc_library {
        name: "bad_example",
        srcs= ["bad.c"],
    }

Test: lunch aosp_arm64-userdebug; make  # runs unit test
Change-Id: If2d3e5d55edccc28d314d99b83b0f54e5c53ac35