Added: Allow users to adjust $TMPDIR clear mechanism on termux exit

The `delete-tmpdir-files-older-than-x-days-on-exit` key can be used to adjust how many days old the access time should be of files that should be deleted from `$TMPDIR` on termux exit. The user can set an integer value between `-1` and `100000`. Set `-1` to delete no files, `0` to delete all files and `> 0` for `x` days. The default value is `3` days. So adding an entry like `delete-tmpdir-files-older-than-x-days-on-exit=10` to `termux.properties` file will make termux delete files older than `10` when termux is exited. After updating the value, either restart termux or run `termux-reload-settings` for changes to take effect.

Note that currently `> 0` will revert back to `0` since deletion is currently broken for empty sub directories and deletion needs to be done based on access time instead of modified time. It will need to be fixed in a later commit. Check `FileUtils.deleteFilesOlderThanXDays()`.

Related issue #2350

termux-shared/src/main/java/com/termux/shared/file/FileUtils.java

@@ -1352,6 +1352,10 @@ public class FileUtils {
                 }
             }
 
+            // TODO: Use FileAttributes with support for atime (default), mtime, ctime. Add regex for ignoring file and dir absolute paths.
+            // FIXME: iterateFiles() does not return subdirectories even with TrueFileFilter for file and dir.
+            // FIXME: Empty directories remain
+
             // If directory exists, delete its contents
             Calendar calendar = Calendar.getInstance();
             calendar.add(Calendar.DATE, -(days));