[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
12 files changed