Support custom "sticky" lite server #5

Currently the code only supports a single hardcoded lightwalletd URL of lite2.hushpool.is. We want to have the same feature of SDL which let's us "sticky" (or force) the wallet to always use the same SDL lite server we configure.

I have tested previously with lite.hushpool.is but haven't verified if it stays on restarts or reverts back. The server could be saved as a preference when changing and then always use that value if a value exists.

I just tested if changing the server reverts on a restart and it did not. Looking at the code here, it already saves a server change in the preferences and will use the preference if changing from the default.
https://git.hush.is/hush/SilentDragonAndroid/src/branch/main/app/src/main/java/cash/z/ecc/android/ui/settings/SettingsViewModel.kt#L27

All preferences are also encrypted.

@jahway603 does this satisfy your stickiness requirement? If you change the server, it will stay on that server. Since there is no logic to try multiple servers, I think "sticky" lite server is currently implemented, correct?

@duke this fits the requirement but I wonder if implementing #1 will change this "sticky" behavior. I'm thinking of how SDL has a setting that will make sure nothing in the app will change the SDL server being used.

@jahway603 yes, if #1 is implemented, we will also need a boolean option for "sticky server" which will prevent the wallet from trying any other server other than the one configured.

@fekt I propose that all options which most users might want to use go on the screen where we currently have the lightwalletd server + port. I think "sticky server" is probably an "Advanced" option, as most users will want the code to try various servers. The "sticky server" option seems to be most used by lightwalletd devs and sysadmins to test a specific server.

I propose that at the bottom of the current options screen, where we have server+port, we have a link to Advanced Options, which most users will not need to touch.