Add rkpd to the list of individually toggleable modules

This is needed because the rkpd module is optional at runtime and so
needs to be optional at build time.

Ignore-AOSP-First: rkpd only exists internally
Bug: 259958590
Test: # Add prebuilts for the rkpd module.
      m MODULE_BUILD_FROM_SOURCE=true droid
      m MODULE_BUILD_FROM_SOURCE=false droid
      # Make sure that they build from sources and prebuilts respectively.
Change-Id: Id0c0e482e2928d57a1bb6ee4f3bcd143ca15122c