Refactor TextUtils.join() and add documentation

Added documentation to clarify what happens in the case of null or
empty arguments.

Change-Id: I694dc0f6a6d95ab6ca7ed95b51d81938618eb75f
Fixes: 36861796
Test: bit CtsTextTestCases:*
Test: bit CtsGraphicsTestCases:android.graphics.cts.FontVariationAxisTest
2 files changed