Simplify GIfDrawable subclassing

[stanmots]

This closes #398

[codecov-io]

Codecov Report

Merging #399 into master will increase coverage by 0.11%.
The diff coverage is 16.32%.

@@            Coverage Diff            @@
##           master    #399      +/-   ##
=========================================
+ Coverage    4.23%   4.35%   +0.11%     
=========================================
  Files          26      27       +1     
  Lines        1322    1332      +10     
  Branches      119     119              
=========================================
+ Hits           56      58       +2     
- Misses       1263    1271       +8     
  Partials        3       3

Impacted Files	Coverage Δ
...rc/main/java/pl/droidsonroids/gif/InputSource.java	0% <0%> (ø)	⬆️
...rc/main/java/pl/droidsonroids/gif/GifDrawable.java	0% <0%> (ø)	⬆️
.../java/pl/droidsonroids/gif/GifDrawableBuilder.java	100% <100%> (+82.05%)	⬆️
...ain/java/pl/droidsonroids/gif/GifDrawableInit.java	15.9% <15.9%> (ø)

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[koral--]

Thanks, this change is available in version 1.2.7-SNAPSHOT.