common/params: ensure Params::readAll() only reads valid param files

[deanlee]

Updates Params::readAll() to explicitly check if each file is a valid param key before reading, instead of relying on util::read_files_in_dir.

This guarantees that only files corresponding to real params are read, making the function truly match its intended definition and semantics.
It improves data integrity and system reliability by preventing unrelated files from being included in the result or written to initdata in loggerd, bootlog.