accounts/abi: implement new fallback functions (#20764)

* accounts/abi: implement new fackball functions

In Solidity v0.6.0, the original fallback is separated
into two different sub types: fallback and receive.

This PR addes the support for parsing new format abi
and the relevant abigen functionalities.

* accounts/abi: fix unit tests

* accounts/abi: minor fixes

* accounts/abi, mobile: support jave binding

* accounts/abi: address marius's comment

* accounts/abi: Work around the uin64 conversion issue

Co-authored-by: Guillaume Ballet <gballet@gmail.com>