[refactor] Move more common methods to base of AttributeConfig<T>

[xndcn]

1. Move more common methods to base of AttributeConfig, decrease source lines from ~750 to ~500
2. Move partial specialization functions into Attributes.cpp, so when it is modified, there is no need to re-compile whole sources which include Attributes.h

[domchen]

@xndcn Declaring specializations in a CPP file causes the PAG decoding to fail on Android. I will revert this pr.
The standard strongly encourages us not to do this, discussed in: https://stackoverflow.com/questions/30530364/explicit-specialization-of-member-function-template-in-source-file Besides, the Attributes.h file have never changed during the last five years.

[xndcn]

@domchen Sorry about it 😂