create capability for specifying custom maps folder

Mark,
Being that I'm working from a fork, the git commands are a bit different, but I did get it to rebase with spatialite_rasterlite2, so they should merge fine.   For prosperity, here's what I had to do:

git checkout mapsDirSettings
git fetch upstream
git rebase upstream/spatialite_rasterlite2
.. fixed strings.xml 
git add geopaparazzi.app/res/values/strings.xml
git rebase --continue
git branch mapsFolderName
git checkout mapsFolderName
git push origin mapsFolderName


added:

        try {
         // mj10777: set start value with existing value
         customFolder=ResourcesManager.getInstance(this.context).getMapsDir().getAbsolutePath();
         sdcardFolder=ResourcesManager.getInstance(this.context).getSdcardDir().getAbsolutePath();
        }
        catch (Exception e) {
         GPLog.error(this, "CustomMapsFolderPreference[invalid getSdcardDir or getMapsDir]", e);
        }

in CustomSdcardPathPreference

        try {
         // mj10777: set start value with existing value
         customPath=ResourcesManager.getInstance(this.context).getSdcardDir().getAbsolutePath();
        }
        catch (Exception e) {
         GPLog.error(this, "CustomSdcardPathPreference[invalid getSdcardDir]", e);
        }