Relax system_server keep rules

Remove explicit keep rules for binder interfaces. These should all be
handled correctly now that we pull in the framework-res manifest
proguard rules.

This saves ~140KB from services.jar.

Test: m + boot + presubmit + inspect services.jar diff
Bug: 300294899
Bug: 246800015
Change-Id: I1d2c41a3be1894a6ffb9599d1a6d88bc6537c442