fix(liveness): add clarification to Android liveness custom credentials provider

[tjleing]

Description of changes

Update Android Liveness docs to specify that custom credentials must be of type AWSTemporaryCredentials as mentioned in this issue.

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Description of how you validated changes

Checklist

• Have read the Pull Request Guidelines
• PR description included
• Relevant documentation is changed or added (and PR referenced)
• yarn test passes and tests are updated/added
• No side effects or sideEffects field updated

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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