Obex library cleanup, first pass.

o Add Android.mk file.
o Add @hide so it is not exposed in the public API. It is not yet in shape.
o Prefer @throws to @exception
o Do not use @version. This is meaningless in Android.
o Prefer classes 'final' unless explicitly allowing inheritance
 (See "Effective Java" item 15)
o Prefer CamelCaps java naming even for acronyms (OBEXHelper -> ObexHelper)
o Use the built-in MD5 library.
o Move ObexConstants into ObexHelper.
o Remove unused variables.
o Prefer stricter access priveleges. Most importantly, avoid public fields.
o Don't use 'import java.io.*'. Name each class explicitly.
o Delete commented out code. If its not used then remove it.