[Cashtab] adjust airdrop totalHolders type

Summary: As per T2373, adjusting the totalHolders type from BigNumber to Int

Test Plan:

• based on the totalHolder logic it's mainly used to determine whether to render the 'At least one airdrop is below the minimum dust' alert, therefore test airdrop scenarios where at least one of the recipients are receiving less than 5.5 XEC output
• regression test various permutations of airdrop options

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11316