Optimized the initState for AnimatedTextKit

[awhitford]

For AnimatedTextKit, the initState was calling _nextAnimation which was calling setState. Technically, setState should not be called from within initState. Refactored the code so that both initState and _nextAnimation call _initAnimation. Initialization is simplified.

[awhitford]

I suspect this should resolve #168.

[codecov]

Codecov Report

Merging #169 (7f9f91b) into master (ef52c3f) will decrease coverage by 1.57%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
- Coverage   91.01%   89.43%   -1.58%     
==========================================
  Files           9        9              
  Lines         445      445              
==========================================
- Hits          405      398       -7     
- Misses         40       47       +7     

Impacted Files	Coverage Δ
lib/src/animated_text.dart	68.13% <50.00%> (-7.70%)	⬇️

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[aagarwal1012]

lgtm

[aagarwal1012]

@awhitford, please do a new release after merging this PR.