Change list

related to: appium/appium#12234

Send both signalStrengh and signalStrength as backword compatibility.
The value should be same.

Types of changes

What types of changes are you proposing/introducing to Java client?
Put an x in the boxes that apply

• No changes in production code.
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Details

signalStrength is also accepted by Appium server after appium/appium-base-driver#308