Provide gradle "TEST_ONLY" flag to completely suppress building the sdk and shims

This is an enhancement to cleanly decouple the test task from the sdk when a flag is set.

* Define a new "TEST_ONLY" property (BUILD_SDK_FOR_TEST=false would set that property to true for backward compatibility)

* Create a new "testShims" task that builds the shims and dependsOn the sdk

* Change the "test" task to conditionally depend on "testShims" iff TEST_ONLY is false

After this change, the goal is that running "gradle -PTEST_ONLY=true test" would not rebuild any of the modules or shims classes nor native libraries.