Adding missing unit tests related to notification listener

[aliabbasrizvi]

Thanks to @mikecdavis for pointing out missing coverage in our unit tests.

Added tests for classes:

• DecisionNotification
• FeatureTestSourceInfo
• RolloutSourceInfo

Also renamed newFeatureVariableBuilder --> newFeatureVariableDecisionNotificationBuilder.

Was unsure if I should add more tests per builder in DecisionNotification.java. Feedback welcome on that.

[coveralls]

Pull Request Test Coverage Report for Build 996

• 10 of 19 (52.63%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 89.39%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core-api/src/main/java/com/optimizely/ab/notification/DecisionNotification.java	9	18	50.0%

Totals
Change from base Build 987:	-0.2%
Covered Lines:	2856
Relevant Lines:	3195

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1000

• 19 of 19 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 89.847%

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Totals
Change from base Build 998:	0.06%
Covered Lines:	2929
Relevant Lines:	3260

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💛 - Coveralls

[mikecdavis]

LGTM. Ironically we lost coverage since you added those null checks. Up to you if you want to add coverage there.