ozone: Initialize a subsystem only if necessary.

Some of the ozone subsystems don't need to be initialized in the GPU
process (e.g. events). So let the ozone-platform know what process the
platform is being initialized for, and initialize only the subsystems
that make sense for that process.

The changes include:
 * Remove SetInstance() from EventFactoryOzone, SurfaceFactoryOzone and
   CursorFactoryOzone. Instead, set the instance from the constructor.
 * Require the platforms to provide one explicit constructor for the UI
   and GPU processes through the OzonePlatform interface.
 * Do not initialize any of the sub-systems in InitializeGPU() in all
   platforms.
 * Move the initialization of ozone-platform into aura::Env::Init.
 * Move the creation of udev-monitor in DeviceManagerUdev from the
   constructor to ScanDevices(), since it needs the message-loop, and the
   message-loop is constructed later.
 * Update EventFactoryEvdev to set the ui_task_runner_ from
   OnDispatcherListChanged(), since this too is created before the
   message-loop.

BUG=361137
R=piman@chromium.org
TBR=spang@chromium.org

Review URL: https://codereview.chromium.org/291473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270903 0039d316-1c4b-4281-b951-d872f2087c98

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