Fix autosave in WalletTool on Windows

Wallet.saveToFile failed to remove original wallet file because WalletTool
left open input stream. This error happened on Windows-specific code path

Stacktrace:
```
Failed to save wallet! Old wallet should be left untouched.
java.io.IOException: Failed to delete canonical wallet file for
replacement with autosave
        at com.google.bitcoin.core.Wallet.saveToFile(Wallet.java:834)
	        at
		com.google.bitcoin.core.Wallet.saveToFile(Wallet.java:863)
		        at
			com.google.bitcoin.tools.WalletTool.saveWallet(WalletTool.java:851)
			        at
				com.google.bitcoin.tools.WalletTool.main(WalletTool.java:362)
```