Refactor AccountBalance to use Balance for transparent funds

[str4d]

Currently AccountBalance stores all unspent transparent funds as a single value, irrespective of confirmation depth:

librustzcash/zcash_client_backend/src/data_api.rs

Lines 211 to 229 in 5a2a670

	/// Balance information for a single account. The sum of this struct's fields is the total balance
	/// of the wallet.
	#[derive(Debug, Clone, Copy, PartialEq, Eq)]
	pub struct AccountBalance {
	/// The value of unspent Sapling outputs belonging to the account.
	sapling_balance: Balance,
	/// The value of unspent Orchard outputs belonging to the account.
	orchard_balance: Balance,
	/// The value of all unspent transparent outputs belonging to the account, irrespective of
	/// confirmation depth.
	///
	/// Unshielded balances are not subject to confirmation-depth constraints, because the only
	/// possible operation on a transparent balance is to shield it, it is possible to create a
	/// zero-conf transaction to perform that shielding, and the resulting shielded notes will be
	/// subject to normal confirmation rules.
	unshielded: NonNegativeAmount,
	}

This is reasonable for the mobile SDKs, which explicitly do not support arbitrary spending of transparent funds (they must always be spent in a carefully-controlled manner, usually being shielded to a shielded address in the wallet first). However, for the zcashd wallet replacement we will need the ability to spend transparent funds, and thus to determine what transparent funds are suitable for selection. We will therefore need to distinguish to users between transparent funds that are available to the wallet for spending, and funds that still require confirmations.

The mobile SDKs can continue to behave as before, by adding up the separate fields to get the same value as they previously reported.