Improve undo behavior on Android

Improve undo behavior on Android

* lisp/simple.el (undo-auto-amalgamate): Update doc string to
describe new amalgamating commands.
(analyze-text-conversion): Make this an amalgamating command by
default, unless a new line has been inserted.  Also, shorten the
undo boundary timer.
* src/textconv.c (really_commit_text)
(really_set_composing_text): Correctly report ephemeral
deletions.
(syms_of_textconv): Fix doc strings.