Improve ability to have large list of instances to choose from

[mrjones-plip]

What feature do you want to improve?
Right now there's a way to select a CHT Instance on the "unbranded" play store app. It is limited though and can not be extended by normal branding techniques:

Describe the improvement you'd like
For large deployments that have many CHT instances, like a national deployment, it would good if we could extend this to have:

• having an external XML file that any APK can use as part of their branding to define a list of CHT instances to show pre-login
• have a way to search long lists of CHT instances per above see Improve server selection usability when choosing an instance from the Settings Dialog #314
• show confirmation dialog when making selection see Improve server selection usability when choosing an instance from the Settings Dialog #314

Describe alternatives you've considered
NA

Additional context
We have an active deployment that is already using this with an alpha APK

[mrjones-plip]

accidentally closed with accidental merge in #306. Re-opening for work being done in #313