This PR adds network mocking for all of our end-to-end (Espresso) tests, including screenshot tests.

• This uses WireMock to power mocking the network requests. you can see the docs here.
• WireMock just runs a small HTTP server on localhost:8080 which mimics the WP API. Request/response configuration is done with the JSON files. I hope to use the same JSON files to mock the iOS tests.
• This URL is injected into FluxC in place of public-api.wordpress.com. Thanks @aforcier for implementing this.
• Images are not currently mocked, and still come from the web. This was proving tricky and I didn't want to block on it any longer. I will follow up on this later.
• There is a new subproject, WordPressMocks which contains all of the configuration and mock files for WireMock. We can make this a subtree later and share the mocks with iOS.

The diff is very big, but it should be not too difficult to review as it is mostly JSON files 😄 You can see it without the JSON files here for easier reviewing. Without the JSON files, this is the diff size 😄

27 files changed, 706 insertions(+), 80 deletions(-)

Still to do:

• Add support for the screenshot tests once Add/enhanced screenshots #9368 is merged.

Once the above is completed, this will be ready for review.

I plan to move libs/mocks to a subtreed repo so the mocks defined here can be shared with iOS.

To test:

1. To esnure you are not using live secrets, and everything is mocked: cp gradle.properties-example gradle.properties.
2. Run ./gradlew WordPress:connectedVanillaDebugAndroidTest --stacktrace.
3. All the tests should pass.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.