Revert^2 "Remove empty source files"

This reverts commit bee0ec26.

Reason for revert: Reland original patch

Keep userial.h to prevent build failures in vendor
builds

Test: m com.android.btservices
Bug: 331817295
Flag: EXEMPT dead code removal
Change-Id: Iadeaaaa65082614f8fb42eae8b538882e7a6af7d