Fix BackgroundColor parsing for lowercase letters

[dpogue]

Platforms affected

iOS

Motivation and Context

The guy who implemented this background colour parsing code forgot that lowercase letters existed when writing the regex 🙄
(aka, me)

Description

Adds support for lowercase hex characters in BackgroundColor preference parsing, along with appropriate tests.

Testing

Added additional test cases with lowercase characters.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change

[codecov-io]

Codecov Report

Merging #1005 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1005   +/-   ##
=======================================
  Coverage   74.91%   74.91%           
=======================================
  Files          13       13           
  Lines        1718     1718           
=======================================
  Hits         1287     1287           
  Misses        431      431           

Impacted Files	Coverage Δ
bin/templates/scripts/cordova/lib/prepare.js	85.00% <100.00%> (ø)

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