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Use a map to track pressed keys. The key for the map is the physical
code that was pressed and the value is a potentially rewritten code.

This allows us to generate appropriate release events when physical
keys are released, while still allowing the event rewriting logic
to work as expected.

Note: Exo clients will always see a release for the same code as
the code that was generated when the physical key was pressed. This
is necessary in order to not break client side keyboard state
tracking and might not match the events that are generated internally.
That is because event rewrite logic is usually not rewriting both the
press and the release events, often just the press event.

Bug: 847500
Test: exo_unittests --gtest_filter=KeyboardTest.OnKeyboardKey
Tbr: hidehiko@chromium.org
Change-Id: I23240304bd377e3855971edd079ade1b537a74f9
Reviewed-on: https://chromium-review.googlesource.com/1112967Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570074}