Added the d8UseRunner option to the plugin configuration to make d8 work since build-tools;29.0.1

[lyubomyr-shaydariv]

build-tools;29.0.1 and later provide the d8.jar library without the Main-Class entry in its manifest, therefore making the current D8Mojo.java unable to work properly. This PR is attempting to fix the behavior.

Items to discuss:

• Changing the behavior:
  • Should 'D8Mojo' provide the flag? (currently suggested implementation)
  • OR should the the plugin resolve the bevavior by assuming the specific build tools versions (e.g., "always lib/d8.jar prior to 29.0.0" and "always d8 in other case")?
  • OR should the plugin peek right into the d8 jar manifest to figure out the preferred behavior?
• I tested the changes only on my Linux Mint machine, and these changes, I believe, cannot work on at least Windows machines (I believe Android SDK for Windows provides d8.bat, "TODO-ed" in the PR). Is there a preferred/cross-platform way of running external scripts in Maven or the plugin?

Please see more at #793.

[lyubomyr-shaydariv]

Closing this one in favor of #796.