Use uint16_t integer types for device IDs and HID usages

Vendor and product IDs are 16-bit values used to identify USB and
Bluetooth devices. Depending on the platform, they may be reported
as integers or strings. This CL modifies all places in Gamepad where
device IDs are stored as strings or signed integers to instead store
them in a minimum-width integer unsigned type (uint16_t).

Similarly, HID usage pages and usage numbers are defined to be
16-bit values. The constants holding these values are changed
to uint16_t.

In a future CL, Gamepad device ID and HID constants will be pulled
out of the platform-specific data fetcher implementations and placed
in a common header.

BUG=786250

Change-Id: I30a0841db91008802331939a86a7b6c3c5f58586
Reviewed-on: https://chromium-review.googlesource.com/1102099
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594495}
18 files changed