Add call-stack reporting methods into Debug

Added two public methods to Debug. These methods return a String
indicating the caller (getCaller()) or callers (getCallers(int depth))
of the calling method. The String indicates the class, method and line
number of the caller(s). Similar to using Throwable.fillInStackTrace()
but much more concise.

Change-Id: I53d0085aa50e4501d28e8eb3ad5b91ef700ac218