Correctly report start and end in extracted text

Correctly report start and end in extracted text

* src/androidterm.c (struct android_get_extracted_text_context):
New field `start_offset' and `end_offset'.  Delete `offset'.
(android_get_extracted_text, android_build_extracted_text):
Replace `offset' with new args `start_offset' and `end_offset'.
(NATIVE_NAME): Set `start_offset' and `end_offset'.
(android_update_selection): Likewise.
* src/textconv.c (get_extracted_text): Likewise.
* src/textconv.h: Update prototypes.