Add a keep rule for SystemUIAnimationLib

This change adds a keep rule for SystemUIAnimationLib so that animation
code can be used by SystemUIClocks at runtime.
com.android.systemui.animation.R$ classes are filtered out to avoid
including transitive resource dependencies.

On panther, this results in these SystemUIGoogle apk/odex size changes:
- SystemUIGoogle.apk:  37628192->37670640 (+41.45KB, +0.11%)
- SystemUIGoogle.odex: 34143312->34197712 (+43.13KB, +0.16%)
- SystemUIClocks.apk:  3867456->1675787   (-2.09MB, -56.67%)
- SystemUIClocks.odex: 29616->21424       (-8 KB, -27.66%)

Test: Build and flash; presubmit.
Bug: 215530220
Change-Id: Ic863d9c788f51132766136a0534d702910f541b8