Support for explicit + was added in this commit:
Support explicit + in Byte, Short, Integer, Long.
Bug: 5239391
Change-Id: I2b25228815d70d570d537db0ed9b5b759f25b5a3
which has been included starting with android-5.0.0_r1. If you have fetched the Git repository, you can verify with:
git tag --contains 6b40837ee3a023bba698c38fd6d6e46ae0065a55
which gives you
android-5.0.0_r1
android-5.0.0_r2
android-5.0.0_r3
...
Even though documentation can give insights into why the change was made (to achieve Java 7 behavior as other answers point out), analyzing the history of the source code gives the most accurate answer to when the behavior changed, since documentation does not necessarily match implementation.