[Systeminfo] Internal implementation changed to use only public native API

[Systeminfo] Internal implementation changed to use only public native API

[Feature] The internal implementaiton was using non-public API of device.h
  library, which makes the hazard of not being supported in future versions.
  To avoid the risk of that, the implementation was improved to use only
  public API. Changed implementation has a guarantee that the API change
  is maintained properly and will not disappear (could be removed only following
  the deprecation and removal procedure).

[Verification] Methods getAvailableMemory() and getTotalMemory() were checked
  with Chrome console.
  TCT passrate of systeminfo module was not changed.

Change-Id: If6f651979f3b3010834f91ff6699b53c770ee760
Signed-off-by: Piotr Kosko <p.kosko@samsung.com>