fix FAB disabled tint color

to be the same as the non-disabled color. at present, the only disabled
FAB is the one in the new transaction activity and it is disabled only
while the transaction is being sent, which is also accompanied with the
fragment (and the FAB) flying to the top, so there is no real need to
use a distinct disabled color

the default disabled tint is black, which is quite ugly, changing from
white-ish