Add Java 6's java.util.ServiceLoader.

New implementation, documentation, and tests, very loosely based on the harmony
code.

We can't run the harmony or jtreg tests because we don't have the infrastructure.
My test just tests the most obvious use cases, on the assumption that at some
point we'll get the other test suites working; quite possibly before anyone ever
wants to use this stuff on Android.

(We might want to switch over existing META-INF/services/ code to use
ServiceLoader. I've raised http://b/2567593 for this.)

Bug: 2497395
Change-Id: I62b0ac4748204555d3ba9356794a72aff4f4f01e
6 files changed