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This cl introduces a SensorDeviceManager for Linux and ChromeOS platforms.

Manager:
1) PlatformSensorProviderLinux owns a SensorDeviceManager that uses
DeviceMonitorLinux to enumerate iio devices and get removed/add udev
events. Once device is enumerated, added or removed, it is identified
and all the necessary information is put to SensorInfoLinux structure
and passed to the provider. If there is no such a device, it is added
to cache. In case of removal, manager checks if there was such a device
and updates it's cache accordingly + notifies the provider about a
removed device, which update own cache.
2) Requests: when a request for a sensor comes, it can be processed
immediately or asynchronously once service is up and running.

Reader:
1) There will be two strategies for a reader. At the moment
only polling read is supported.
2) The reader doesn't search for sensor files any more, those are passed
to it in SensorInfoLinux structure.

Unittests:
1) In order to make it possible to test the whole path, I've made
SensorDeviceManager's methods virtual, which allowed me to
mock and override them.
2) Unittests implement own udev methods that just read values from
files. If read is successful, the manager adds this type of sensor
associated with the files that have been read to it's cache and notifies
the provider.

BUG=606766

Review-Url: https://codereview.chromium.org/2533793002
Cr-Commit-Position: refs/heads/master@{#437550}