feat: Added caching for OptimizelyConfig object

[fayyazarshad]

Summary

Updated the OptimizelyConfig implementation to always return cached object. OptimizelyConfig object will only be updated only when new ProjectConfig is received.

Test Plan

Added Unit tests

[coveralls]

Pull Request Test Coverage Report for Build 1332

• 7 of 24 (29.17%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 89.278%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core-api/src/main/java/com/optimizely/ab/Optimizely.java	4	12	33.33%
core-api/src/main/java/com/optimizely/ab/optimizelyconfig/OptimizelyConfig.java	0	9	0.0%

Totals
Change from base Build 1326:	-0.2%
Covered Lines:	26229
Relevant Lines:	29379

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💛 - Coveralls

[jaeopt]

LGTM

[mikecdavis]

LGTM with one request. Let's add the OptimizelyConfigManager to the constructor as a @Nullable parameter and handle the default from the builder. I prefer to keep the constructor devoid of business logic.