[Android] Theming Customization Improvements

[nizmaylova]

Goal
Simplify the theme configuration for builds of the new openedx mobile app

Slack: #wg-product-mobile
Requirement Definition: Not Drafted

### Tasks
- [ ] https://github.com/openedx/openedx-app-android/issues/193

[farhan-arshad-dev]

Hi @k1rill

I have identified a code improvement related to theming with config pull request. Propose to implement a check to verify if the platform directory does not exist, use the default `srcDir/OpenEdx' will be considered. This enhancement will be beneficial for the community, especially for those who wish to utilize default colors or themes during development.

[marcotuts]

Added final open task to Q2 bugs epic, marking this closed.