[SysApps] Implement the Device Capabilities getMemoryInfo()

This is probably the easiest to implement and the most portable API. It could be
implemented without a "provider", but let's just keep using the pattern to make
the implementation more coherent.

We are using the Chromium abstractions, so it should work on all platform
supported by Chromium with no need of provider abstractions.
[SysApps] Add unit tests and browser tests for getMemoryInfo()
Merge pull request #1376 from tmpsantos/devcap_memory

[SysApps] Implement the Device Capabilities getMemoryInfo()