Move remaining KeyCode files to platform/

This is a reland of r164107 (which was reverted due to an Android build error).
https://codereview.chromium.org/111403004

There was a missing #include in KeyCodeConversionAndroid.cpp which prevented a function from being exported from the platform component.

BUG=297477,329743

Review URL: https://codereview.chromium.org/106103009

git-svn-id: svn://svn.chromium.org/blink/trunk@164363 bbb929c8-8fbe-4397-9dbb-9b2b20218538