Move creating the source jar setup to SupportLibraryPlugin.java

This is the second CL in the series of cleaning up build.gradle files
in support library. This change moves the creation of the task to build
source jars to SupportLibraryPlugin class.

Bug: 33845666
Test: ./gradlew clean createArchive still generates source jars.
Change-Id: Ie8c73c2b71248cb129884be3312b47eeaa6ffbcf
26 files changed