In RustBackend.proposeShielding, when transparent_receiver is null select the first taddr that has sufficient funds to shield

[daira]

@str4d:

consider altering the Rust FFI backend to filter by threshold and then select the first taddr after that (so it doesn't get stuck always trying to select a taddr with insufficient funds to shield).

zcash-android-wallet-sdk/backend-lib/src/main/rust/lib.rs

Lines 1784 to 1798 in 3558ecf

	let from_addrs = if let Some((addr, _)) = transparent_receiver.map_or_else(||
	if account_receivers.len() > 1 {
	Err(anyhow!(
	"Account has more than one transparent receiver with funds to shield; this is not yet supported by the SDK. Provide a specific transparent receiver to shield funds from."
	))
	} else {
	Ok(account_receivers.iter().next().map(|(a, v)| (*a, *v)))
	},
	|addr| Ok(account_receivers.get(&addr).map(|value| (addr, *value)))
	)?.filter(|(_, value)| *value >= shielding_threshold) {
	[addr]
	} else {
	// There are no transparent funds to shield; don't create a proposal.
	return Ok(ptr::null_mut());
	};

Note that essentially the same code is implicated here and in Electric-Coin-Company/zcash-light-client-ffi#171 . It may be preferable to move the logic for selecting which taddr to shield from by default into librustzcash, so that it can be shared.