[Android] Update SendKeyEvent docs to point to constants.keyevent
[Android] Update SendKeyEvent docs to point to constants.keyevent

Point the module containing named constants for keycode values.

BUG=

Review URL: https://codereview.chromium.org/1134773002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#329129}

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