use BufferedInputStream in readTextFile

With FileInputStream, using assumption of bytes read (as
returned by read function) not matching buffer size as EOF is
not correct, we hit this when reading from /proc in some cases.

BufferedInputStream's behavior matches that assumption due to
its implementation.

Bug: 7342759
Change-Id: Ibb06dbe1fbe33f6f880524f7555ff5542889ea50