[codec importance] Add aconfig feature flags

Test: Manual:
 - Test flag can be called in both codec frameworks (C++)
   and CTS (java)
 - Test flag can be flipped by tooling
  - adb shell device_config put codec_fwk\
      com.android.media.codec.flags.codec_importance true

Bug: 297929011
Change-Id: Iedf841d726ec596cca62fd446ffcd2b2185c6d07