Request: make some methods public

[j-garin]

Unable to create custom ItemDecoration for FlexboxLayoutManager because getPositionToFlexLineIndex has package private access modifier. The same goes for FlexLine's mFirstIndex and mLastIndex.

Please, consider making them public or making public getters.

Version of the flexbox library

1.1.0

[kexuejin]

I need this function too, this is necessary for custom ItemDecoration.