Adds Watch Face Push validation library snippet

[garanj]

Adds snippets for Watch Face Push validation library.

Created a separate directory for these, as the library is not an Android library, but is used in the build process by developers creating Watch Face marketplace apps.

The library itself cannot run on Android.

[snippet-bot]

Here is the summary of changes.

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• watchfacepush/validator/src/main/kotlin/com/example/validator/Main.kt:22, tag android_examples_wfp_validation

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