Added unit tests to PropertyFile and const getters

Added unit tests to PropertyFile and const getters
 - testSampleInfoFromFile()
 - testGetOptional()

    src/util/PropertyFile.cpp
    src/util/PropertyFile.h
    src/util/TestPropertyFile.h
Added TestPropertyFile::testInfoDuplicateEntry

Added TestPropertyFile::testInfoDuplicateEntry
    common_build.py
    src/Android.mk
    src/util/TestPropertyFile.h
Added PreferenceHelper Class w/tests

Added PreferenceHelper Class w/tests
This makes it easy to have multiple levels of preferences.
Default, system, and user defined preferences.
The priority is User -> System -> Default.

User is always king, so all preferences are written first to Default, if
anything is platform specific, override that entry in System.
Other than that, whatever the user has been able to specify trumps.

    src/util/PreferenceHelper.cpp   (+)
    src/util/PreferenceHelper.h     (+)
    src/util/TestPreferenceHelper.h (+)