Add unit tests verifying the webview package versioncode validation

We recently added functionality for ensuring that a webview package
cannot be used unless its version code is at least as high as the
minimum of the version codes of the default webview packages.
This CL contains unit tests for verifying this functionality.

Bug: 28050853
Change-Id: Iaf6d5999e420a7eddc4df06aaeefd68ee545422e