Tune the user settings about specifying gradle distribution

[testforstephen]

Signed-off-by: Jinbo Wang jinbwan@microsoft.com

Close #1495

[testforstephen]

// @akaroml @jdneo @Eskibear

[Eskibear]

java.import.gradle.version: Use Gradle from the specific version if the Gradle wrapper is missing or disabled.

Just to confirm that, if java.import.gradle.wrapper.enabled is true, we don't honor java.import.gradle.version, right?

[testforstephen]

Also if your project has gradle wrapper, then we don't respect java.import.gradle.version.

The extension follows the order below to find the gradle distribution:

1. Check whether gradle.wrapper.enable preference is enabled and your workspace contains gradlew script.
2. Check gradle.version perference.
3. Check gradle.home preference and GRADLE_HOME environment variable.

[fbricon]

Thanks @testforstephen