Add Debug Credentials

[theck13]

Fix

Add the two items to the gradle.properties file so that the email and password fields will be automatically populated for debug builds. If the loginEmail and loginPassword items are set in gradle.properties, they will be inserted into the fields for the login flow. Otherwise, the fields will be empty.

Test

0. Clear app data.
1. Tap Sign Up button.
2. Notice Email and Password fields are empty.
3. Tap back arrow in app bar.
4. Tap Log In button.
5. Tap Log In with Email button.
6. Notice Email and Password fields are populated with loginEmail and loginPassword values from gradle.properties.

Review

Only one developer is required to review these changes, but anyone can perform the review.

Release

These changes do not require release notes.

[peril-automattic]

You can test the changes on this Pull Request by downloading the APK here.

[anitaa1990]

Looks good!