Fixed Unknown Caller Issue and Other Bug Fixes

[eopeter]

I wrote the original Twilio Voice Flutter Plugin that this Plugin was "forked" from. I deprecated mine and is this has more contributors and has advanced a lot further. This PR has these changes:

• I fixed issue where the Shared Preference Id is different across Activities which caused looking up the caller return not found and register Unknown Caller
• Allow registering a client to modify the SharedPreference even if already exists
• Removed .idea folder and added .gitignore entries for .idea and .vscode
• Upgrade Gradle and Compile SDK Versions
• fixed incoming call typo from incomming to incoming

[diegogarciar]

Thank you for creating this PR! I'll be reviewing this thoroughly during the week.

[diegogarciar]

Works great, I just think we need to resolve the comment before I can merge!