Fix ProxySettingsFragment crash

This patch addresses issue #396 (closed)

Bug report:

• App crashes when trying to enter a password for proxy settings

How to reproduce the error:

• Settings > Network > Proxy settings > Select Password

Expected:

• Successfully input password

Cause:

• A NullPointerException error occurs when the code tries to mask a null password value and output it

Fix:

• Change password masking method referring to TransformationMethod and PasswordTransformationMethod

I hope this patch contributes to a more secure use case of LibreTorrent!

Pre-launch Checklist

• [v] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• If you made changes to the code:
  • I added new tests to check the change I am making or feature I am adding.
  • [v] All existing and new tests are passing.