Refactor example app build config keys

[jreij]

Description

Improvements to the example app build config (related to #1486):

• Rename default.local.gradle to example.local.gradle.
• Fail purposefully on first build to make sure the example.local.gradle file is copied and its placeholders are replaced with the actual values.
• Rename API_KEY to AUTHORIZATION_HEADER and redact it from logs.
• Make the authorization header and merchant account optional for implementations that handle those on the server.
• Move shopper reference out of build config and add it as a default value to settings.
• Display default value of merchant account in settings screen.

How to migrate

If you already had set up the example app, this PR will cause your build to fail. To migrate go to example-app/local.gradle and follow these steps:

• Rename API_KEY_HEADER_NAME to AUTHORIZATION_HEADER_NAME.
• Rename CHECKOUT_API_KEY to AUTHORIZATION_HEADER_VALUE.
• Remove SHOPPER_REFERENCE line. It will now default to "test-android-components", you can change that value in the settings screen if you want.
• Remove unnecessary quote symbol escaping: "\"SOME_VALUE\"" can be replaced with '"SOME_VALUE"'.

Checklist

• Changes are tested manually

COAND-831

[araratthehero]

Looks good to me!

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[jreij]

@OscarSpruit all good from my side but I can't approve my own PR 😅 If you are happy with my changes from the first 7 commits please approve.