Make services.core the final priorityboosted jar

Other module besides services depend on services.core, and after
I417409281c928ea667d937090d2a0d9d72a449a2 they were getting
the non-priorityboosted version of the jar.  Make services.core
a java_library that contains the output of the priorityboosting
java_genrule.

Test: m services
Change-Id: Ic9ca1b8f43f87b0d7d36b53bbc5136a331490c3e