feat(splash): Support SplashScreenBackgroundColor preference

[dpogue]

Platforms affected

Android

Motivation and Context

Adds support for the more general SplashScreenBackgroundColor preference.

Description

Sets the splashscreen background based on the following options (in order):

• AndroidWindowSplashScreenBackground preference
• SplashScreenBackgroundColor preference
• BackgroundColor preference
• #ffffff hardcoded

Testing

Didn't seem like there were any existing unit tests for the splashscreen stuff, so I didn't update any tests.

• [x] This still needs to be tested manually in an Android project.

Checklist

• [x] I've run the tests to see all new and existing tests pass
• [ ] I added automated test coverage as appropriate for this change
• [ ] I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 72.15%. Comparing base (b773ae4) to head (99532c2).

Files	Patch %	Lines
lib/prepare.js	0.00%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1700 +/- ## ========================================== - Coverage 72.35% 72.15% -0.20% ========================================== Files 23 23 Lines 1834 1839 +5 ========================================== Hits 1327 1327 - Misses 507 512 +5 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.