Fix invalid json when product variables have quotes

Go's json encoder will escape quotes with backslashes.
But we put the encoded json into a Starlark string
literal, which will evaluate the \" into just a regular
quote, so they won't be escaped when the json.parse()
gets to them.

Escape all backslashes so this doesn't happen.

Test: lunch sdk_phone_x86_64-userdebug; b build //build/bazel/examples/apex/minimal:build.bazel.examples.apex.minimal
Change-Id: I473e6c42968fcf73d47dec61670956a7ac9a6c88