Add a services.impl intermediate target

Introduce a new static library, services.impl, that contains all static
deps for for services. This target can be used as a proxy `libs` dep
by downstream system server jars for linking, allowing those downstream
jars to be used as tracing references when generating the final services
target keep rules.

Bug: 212737576
Test: m + presubmit
Flag: EXEMPT build target refactor
Change-Id: I16733c8127b71a6449adf18bbc3b2e720556ac58