Replace ktfmt.py hook by built-in hook in ThemePicker/

This CL replaces the ktfmt.py hook of this repository by the builtin
ktfmt hook. This will allow to delete our custom hook and provide
features like automatic fixup (see http://screen/5uXuuPgs7oL83c2).

Note that this also means that this CL is upgrading ktfmt from v0.43 to
v0.49, so it is expected to see some minor formatting diffs on existing
files after this CL is submitted.

Bug: 342153000
Test: N/A
Flag: NONE Repohook Update
Change-Id: I6bee51bc6c695c512d490be2ed8103f78838cf57