Add codecov support

[henriquenfaria]

• Add code coverage badge to README
• Add jacoco-android plugin
• Update Robolectric to version 3.8
• Add truth to support fluent assertions
• Refactor current automated tests

[googlebot]

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[henriquenfaria]

I signed it!

[googlebot]

CLAs look good, thanks!

[samtstern]

@henriquenfaria thank you! This looks great.