Relax system server proguard keep rules

SystemService classes are now explicitly referenced if they are
bundled in services.jar, so we need only ensure that their constructors
are kept.

This paves the way for build-time stripping of SystemService types that
are guarded by build-time flags. However, this change should be a no-op
as all such references are assumed reachable.

Bug: 203143243
Test: m + compare services.jar and ensure no SystemService changes
Change-Id: I97eeed3a275d272ae0e00916f52f84a17e65325d