Include xml attribute strings from res-public to sourceSet.

[pengdev]

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Summary of changes

We use public.xml for indicating which attributes are private vs public (more context here). With using the public.xml configuration, we should be able to find a public.txt file in the resulting aar:

When building a library, the Android Gradle plugin gets the public resource definitions and extracts them into the public.txt file, which is then packaged inside the AAR file.

This is currently broken since we didn't configure the src/main/res-public to the sourceSet.

This PR includes public xml attribute strings from res-public to sourceSet.

User impact (optional)