ash: Add VKEY_SLEEP as a system key

On sarien device, lock key has VKEY_SLEEP. As ARC does not expect
VKEY_SLEEP to be delivered, it enters bad state after receiving it.
(See #1 of the bug.)

Similar to VOLUME_UP/DOWN and POWER, SLEEP key should be considered as
a system key, and should not be forwarded to ARC. In the long term, we
may have to fix ARC side as well.

This is M77 ReleaseBlock-Stable.

TEST=manual(On sarien device, open Play Store and press lock key. Close
and open Play Store again, then Close and open Play Store again and
confirm it confirm it works as intended.)
BUG=b:140127435

Change-Id: I9e238b39dd283fd4148cdafeff2da05adde8dda0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1792213
Auto-Submit: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694797}

ash/accelerators/pre_target_accelerator_handler.cc

@@ -33,6 +33,7 @@ bool IsSystemKey(ui::KeyboardCode key_code) {
     case ui::VKEY_VOLUME_DOWN:
     case ui::VKEY_VOLUME_UP:
     case ui::VKEY_POWER:
+    case ui::VKEY_SLEEP:
       return true;
     case ui::VKEY_MEDIA_NEXT_TRACK:
     case ui::VKEY_MEDIA_PLAY_PAUSE: