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.

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

Checklist

• 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 ⚠️

Additional details and impacted files

@@            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     

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[erisu]

Code LGTM.

Also tested using the three preferences with Cordova's default template app. Confirmed order of priority.