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The iDroid:con gamepad has multiple modes. This CL adds mappings for the
two modes where it reports as a gamepad.

Also use axes_used rather than axes_length for distinguishing modes on
Linux.

BUG=1073130

Change-Id: I2741de8097155f36eb7b6f60f69fc766b1462707
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2204184
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773227}