Add guidance about dark mode to the Android style guide

[seanlip]

Platform
Oppia Android

Describe the request
Currently, we need to provide mocks for Android dark mode when submitting Android design projects. There is some guidance on how to do that here but it is not very approachable. The aim of this issue is to make this process easier and incorporate an easy-to-use section in the Android style guide for how to apply dark mode to an existing Android design.

We might also want to look at previous design mocks and compile the dark-mode colours from there to add to the design system file.

Note: To do this project, you will need a phone that displays the "dark mode" option.

Context for the request
This is high-priority because it is a key part of all Android design projects.

Information about deadlines/priorities
This should be done ASAP because it affects all Android design projects.

DESIGN APPROVAL CHECKLIST -- DO NOT EDIT.

• UX Design approved (Android mocks should include dark mode)
• Product approved

For this design project to be completed, it needs approval from UXD and PM. Please tag the following reviewers when your project is ready (at least one reviewer for each of the 2 bullet points below):

• UXD: N/A (for design style guide projects)
• PM: @seanlip