Android : make map location changeable from within the app

[jandegr]

To cope with device specific configurations and user preferences, we need a user-friendly way to change the location where the app can find and should store maps, in an Android compliant way, so gui-internal is exlcuded because it violates at least 2 Android rules.

[jandegr]

Attached a diff of a first attempt using ​https://github.com/vaal12/AndroidFileBrowser as activity to choose a path.

Result : allows to choose a medium / path in-app where Navit looks for maps, stores downloaded maps and looks for a custom navit.xml as well.

TODO : -implement some rules for when to allow and when to force a user to select a medium -check for usability of the medium / path at each startup -add some filter to remove asec, cd-rom, obb, shell and such from the list of selectable paths. -let the filebrowser show which media are removable and which are internal or emulated.

By default the paths in navit.xml are relative now but *core users, addicted to their beloved navit.xml, can still work in the same way as they used to, with absolute paths.

Only works for maps, if you want to use the map tag it is expected that you know its exact location as well.

The relative paths work nicely in combination with the relative path option in the textfile maps (as already used in #1285). And I now have distinct folders for testing and real usage, each holding another set of maps and another navit.xml.

[jandegr]

in some way relates to #1217, #1218 and #1286

[kazer]

I had an issue with this patch similar to the one i had with your patch from #1285