Fix failing unit tests and update libraries/deps to latest versions

[danielworkman]

• Updated the Powermock/Mockito library to version 2.0.9 which gets the failing unit tests passing again. There is however a new warning emitted as a result but this appears to be ignorable:

  WARNING: An illegal reflective access operation has occurred
  WARNING: Illegal reflective access by org.powermock.reflect.internal.WhiteboxImpl (file:/Users/Dan/.gradle/caches/modules-2/files-2.1/org.powermock/powermock-reflect/2.0.9/4bb9ed43e5221926fb86cae44b445de110a51d05/powermock-reflect-2.0.9.jar) to method java.lang.Object.clone()
  WARNING: Please consider reporting this to the maintainers of org.powermock.reflect.internal.WhiteboxImpl
  WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
  WARNING: All illegal access operations will be denied in a future release

• Updated target SDK to latest
• Updated a number of other dependencies to their latest version
• [Replaced deprecated jcenter() with mavenCentral()](https://blog.gradle.org/jcenter-shutdown#:~:text=The%20jcenter()%20method%20will,may%20only%20publish%20new%20versions.)
• Commented out a stub that was throwing an error under the latest version of Mockito