[java-sdk] Adds checkstyle and bug finder

[artursouza]

• Adds a checkstyle to Java SDK and make build fail due to style violations.
• Adds SpotBugs and make build fail due to violations.
• Fixes existing violations.
• Generated code should be excluded from static analysis.

[yaron2]

Can we close this?

[LMWF]

Checkstyle is done, static analysis is not complete (spot bugs).