bmpblk: Update dropdown arrow icons

Rename ic_dropdown to ic_arrow_down and replace the triangle with the
keyboard_arrow_down chevron from Google Symbols (weight 400, grade 0,
optical size 24). Add ic_arrow_up with the keyboard_arrow_up chevron for
the expanded state. Both icons share the new ic_arrow style. This
matches the dropdown trigger in the new firmware UI design. Keep the
fill white so that depthcharge can recolor the icons at draw time.

BUG=b:551743258
BRANCH=none
TEST=LOCALES=en BOARD=asurada make
TAG=agy
CONV=4d411a1c-369f-4fd3-ac8a-7bd726afa3b5

Cq-Depend: chromium:8485908
Change-Id: Idb44c8164c75a5077b3ec6505ba30e80362abb8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bmpblk/+/8482776
Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-by: Chen-Tsung Hsieh <chentsung@chromium.org>
Tested-by: Yu-Ping Wu <yupingso@chromium.org>
GitOrigin-RevId: 79b3f5a53d632a3c170f328390285520e8130cd8