Tramp adb fixes, found during test campaign.

Tramp adb fixes, found during test campaign.
* net/tramp.el (tramp-call-process): Improve trace message.
(tramp-handle-insert-file-contents): Trace error case.

* net/tramp-adb.el (tramp-adb-file-name-handler-alist)
<insert-directory>: Use `tramp-handle-insert-directory'.
(tramp-adb-handle-insert-directory): Remove function.
(tramp-adb-send-command-and-check): New defun, replacing
`tramp-adb-command-exit-status'.  Change all callees.
(tramp-adb-handle-file-attributes)
(tramp-adb-handle-directory-files-and-attributes): Use it.
(tramp-adb-ls-output-name-less-p): Use
`directory-listing-before-filename-regexp'.
(tramp-adb-handle-delete-directory): Flush also file properties of
the truename of directory.
(tramp-adb-handle-file-name-all-completions): Add "./" and "../".
(tramp-adb-handle-file-local-copy): Make the local copy readable.
(tramp-adb-handle-write-region): Implement APPEND.
(tramp-adb-handle-rename-file): Make it more robust.  Flush file
properties correctly.
(tramp-adb-maybe-open-connection): Set `tramp-current-*'
variables.  Check for connected devices only when needed.