Don't clear devices during gamepad enumeration

In GamepadPlatformDataFetcherLinux, when enumerating devices with
EnumerateSubsystemDevices, we incorrectly clear the device list. Since
we enumerate multiple subsystems, this causes us to lose all devices
except those found in the last subsystem. This CL instead clears the
device list before enumeration begins.

This CL also combines some subsystem strings that were duplicated.

BUG=749295

Change-Id: I905b75ba37e1e53cbd67777ed7e07c2a66df4073
Reviewed-on: https://chromium-review.googlesource.com/865807
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529859}
3 files changed