Resolves 'gradlew dist' Errors on execution of command consecutively

[rupeshkumar22]

Fixes #687

Resolves both gradlew dist and gradlew build Errors on executing the command one by one.

Why the issue arises?
Because core.jar, vr.jar, etc. files are not getting generated because the Jar tasks are getting skipped. The reason for skipping of jar task is because it is UP-TO-DATE or the source files are not getting changed. This can be identified by gradlew dist -info which says the Jar task is skipped(while executing the command again without modifying the source files)

When executing gradlew dist -info for the first time it shows the following-

> Task :core:jar
Caching disabled for task ':core:jar' because:
  Build cache is disabled
Task ':core:jar' is not up-to-date because:
  Output property 'archiveFile' file *\processing-android-android-0402-v4.5.0a3\core\build\libs\core.jar 
has been removed.
*\processing-android-android-0402-v4.5.0a3\core\build\libs\core.jar
(Thread[Execution worker for ':',5,main]) completed. Took 0.396 secs.
:core:sourceJar (Thread[Execution worker for ':',5,main]) started.

Executing the same command for the second time without changing the source files shows the following-

> Task :core:jar UP-TO-DATE
Caching disabled for task ':core:jar' because:
  Build cache is disabled
Skipping task ':core:jar' as it is up-to-date.
:core:jar (Thread[Execution worker for ':',5,main]) completed. Took 0.001 secs.
:core:sourceJar (Thread[Execution worker for ':',5,main]) started.

This issue is the same for the libraries AR and VR.

Solution with this PR
This PR applies 'if conditions' to check the existence of necessary files i.e core.jar, vr.jar, etc. Do the copy/move operations if files exist otherwise no need to copy or move the files as we know the files do not exist and processing-core.jar, ar.jar, etc. are already UP-TO-DATE(jar task get skipped in this case).

[codeanticode]

Cool, thanks!

[rupeshkumar22]

Thanks for the review! @codeanticode