[System.Core/Android] Fix standalone unit tests.

[System.Core/Android] Fix standalone unit tests.

Commit 5564dad1 broke the self-test (for testing on the developer
machine, not an Android target). Fixup the comment so that appropriate
data is grabbed and used for testing, and improve
AndroidTimeZones.GetDefaultTimeZoneName() so that the default timezone
ID can be specified via the __XA_OVERRIDE_TIMEZONE_ID__ environment
variable for testing.

Finally, demonstrate TimeZoneInfo.GetUtcOffset(DateTime) use so that I
can get better insight into the timezone data stored within the
database:

	$ __XA_OVERRIDE_TIMEZONE_ID__=America/New_York ANDROID_ROOT=`pwd` ANDROID_DATA=`pwd` mono --debug tzi.exe --offset=1969-01-01
	Using DateTime Offset: 1/1/1969 12:00:00 AM
	DB type: System.AndroidTzData
	name=Africa/Abidjan                           Africa/Abidjan                          From Offset: 00:00:00
	name=Africa/Accra                             Africa/Accra                            From Offset: 00:00:00
	name=Africa/Addis_Ababa                       Africa/Addis_Ababa                      From Offset: 03:00:00
	...