MainActivity: run connection test on another thread

MainActivity: add a button for testing connection

SFTPDocumentsProvider: set dummy flag

we do not support any of the flags yet, but this should prevent
DocumentsUI with docinfo from crashing

SFTPDocumentsProvider: unify stat filling

SFTPDocumentsProvider: make title url-ish

SFTPDocumentsProvider: append mount path to title

SFTPDocumentsProvider: do not add port to title if canonical

SFTPDocumentsProvider: add summary

SFTPDocumentsProvider: correct connection resetting

SFTPDocumentsProvider: proper error handling

SFTPDocumentsProvider: refactor toasting

reformat code

fix various lints

build: bump sshlib to 2.2.20

seekable reader, bump min sdk level to 26

build: modernize toolchain

README: we support custom path now

don't reuse SFTPv3Client, it's not costy and this enables concurrency

ReadTask: always try to close the file

add mount path settings