Update target API level to allow submitting app updates after November 1, 2018

[jwalgran]

Blog post - "Improving app security and performance on Google Play for years to come"
https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html

Migration Guide
https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html

[maurizi]

I believe there is a significant amount of work involved in updating the target API level. It has been some time since I looked into it for the OTM app, but at a minimum I think we'd want to do the following:

• Upgrade the build scripts to the latest gradle plugin version (as part of this we'd want to migrate from RetroLambda to the new built-in lambda support)
• Update the styling app-wide from Holo to Material Design
• Update the code to support the new runtime permissions model