Add memory, battery and storage info to mobile context

[matus-tomlein]

We want to extend the range of device information sent as context along with events to automatically include memory, storage and battery properties. Concretely, we identified the following properties to include:

• memory
  • physicalMemory – Total physical system memory in bytes
  • systemAvailableMemory – Available memory on the system in bytes (Android only)
• battery
  • batteryLevel – Remaining battery level as an integer percentage of total battery capacity
  • batteryState – Battery state for the device
• storage
  • availableStorage – Bytes of storage remaining
  • totalStorage – Total size of storage in bytes

The mobile_context schema version 1.0.2 provides properties for all the information (see issue here).

We decided to add the properties under the same tracker configuration for platform context (platformContext) as done with previous information about device platform (which included network status). We do not expect the new information to incur significant overhead in computation or space and so there is no need to introduce new configuration options.