Exposed the constructor of Note class

Exposed the constructor of Note class

Additionally, defined the NoteMap.getNote method which returns a Note
instance.  These changes were necessary to enable implementation of
the NoteMerger interface (the merge method needs to instantiate a
Note) and to enable direct use of NoteMerger which expects instances
of Note class as its paramters.  Implementing creation of code review
summary notes in Gerrit [1] will make use of both of these features.

[1] https://review.source.android.com/#change,20045

Change-Id: I627aefcedcd3434deecd63fa1d3e90e303b385ac
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>