fix #2222 move default backgroundColor to initialize()

[liuxuan30]

fix #2222
move default backgroundColor to initialize() as initWithCoder also needs it
I checked initialize() is only called in init(), so it seems safe to move there, to avoid duplicated code.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1ed1561). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #2228   +/-   ##
=========================================
  Coverage          ?   18.44%           
=========================================
  Files             ?      124           
  Lines             ?    14619           
  Branches          ?        0           
=========================================
  Hits              ?     2696           
  Misses            ?    11923           
  Partials          ?        0

Impacted Files	Coverage Δ
Source/Charts/Charts/ChartViewBase.swift	24.44% <100%> (ø)

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