Add note to Changes.md about sysprop changes

# Build System Changes for Android.mk Writers

## Stop referencing sysprop_library directly from cc modules

For the migration to Bazel, we are no longer mapping sysprop_library targets

to their generated `cc_library` counterparts when dependning on them from a

cc module. Instead, directly depend on the generated module by prefixing the

module name with `lib`. For example, depending on the following module:

```

sysprop_library {

    name: "foo",

    srcs: ["foo.sysprop"],

}

```

from a module named `bar` can be done like so:

```

cc_library {

    name: "bar",

    srcs: ["bar.cc"],

    deps: ["libfoo"],

}

```

Failure to do this will result in an error about a missing variant.

## Gensrcs starts disallowing depfile property

To migrate all gensrcs to Bazel, we are restricting the use of depfile property