[Media ML] Expose config values for OEMs to set custom classes

Expose internal config values that OEMs can use to customize
media service code as SystemApi in order to access it from
mainline module.

Bug: 178774406
Test: Cherry-pick ag/9881175, build/flash, and call
      R.string.config_customMediaKeyDispatcher to test that config
      value has been overlayed.
Change-Id: Ia23af4ce4fa20b06f70fc3d964c7f3db0bd35325