Providing offset and density breaks image comparison on android23/24 by not excluding the software buttons

[zbranzov]

Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project:

• CLI: 6.2
• Cross-platform modules:6.2
• Android Runtime:6.2
• iOS Runtime:6.2
• Plugin(s):6.1.2

Describe the bug
Providing offset and density breaks image comparison on android23/24 by not excluding the software buttons

To Reproduce
Add offset and density values in the capabilities. Compare two screens with software buttons with some change on the buttons.

Expected behavior
Providing offset and density should not affect the successful screen comparison otherwise it is a breaking change for those APIs.

Sample project
https://github.com/NativeScript/nativescript-ui-chart/pull/137/files