Export PluginCoreLib proguard flags

Ensure runtime annotations are kept for any downstream clients
that reference the plugin library dynamically. In practice this
should be a no-op for such clients that have had to manually bundle
those proguard rules.

Bug: 212623640
Test: presubmit
Flag: NA
Change-Id: If9f0cd8955b4d179338ec8d76aaeb7d6ffb2c511