Prepend header jar for sharded javac compilation

When compiling without sharding javac will prefer classes built from
source over classes in the classpath.  Compiling with sharding puts
the header classes for all the sources onto the classpath, including
the sources that are not part of the current shard.  Emulate the
behavior when building without sharding by putting the header jar
first on the classpath so classes are resolved from the rest of
the sources before the real classpath.

Also switch to using the header jar without static libs dependencies
merged in, as those will already be present in the classpath.

Test: m framework-minus-apex
Test: m services.core
Test: TestSharding
Change-Id: Id11b39c15d89ce70da27d746a5c7e6b603c3c0f9