[Cashtab] Replace step with constant
Summary: Replace fixed .01 step parameter with a constant, the lowest unit of BCHA the app will send
Test Plan: npm start and go to Send screen. Type a number in the input and use the scroll wheel. Observe it increments by 0.00001 instead of 0.01. This solves bug where mobile users saw error msg on attempting to send more than 2 decimal places.
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D8971