Use variable fonts for Noto Sans Khmer

Update fonts.xml to use NotoSansKhmer-VF.ttf.

The weight range of Khmer is also extended to have 9 weight instances
(three more than Roboto with 200, 600 and 800).

The width range for Khmer is left alone partly because of an apparent
lack of support in fonts.xml for non-normal width fallback fonts.

Note that Noto Sans Khmer UI is left alone because of a baseline issue.

BUG: 22667753
Test: See the QA instruction in comment 10 of the bug.
Test: fontchain_lint
Change-Id: Idf6d4fec86822663ac08feb8979edacbc6c709a0