Fix docs build breakage in #TextView_letterSpacing

This patch removes the @hide annotations and public reference from the
new letterSpacing attribute, so the docs will build. It's still not an
ideal state because a test app can't find the attribute, but at least it
unbreaks the build.

Change-Id: I5425fbe3d63f7a759fabe43bd2136196c7112eed
3 files changed