[Cashtab] [p2 rename smallestDenomination] Rename toSmallestDenomination to fromXecToSatoshis

Summary:
T2605

Depends on D11850

part 2 of stacked diff to rename fromSmallestDenomination and toSmallestDenomination functions

Renames toSmallestDenomination to fromXecToSatoshis

Test Plan:
npm test
grep -r 'toSmallestDenomination' src/ and verify no output

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11851