Decouple libstagefright from Codec 2.0

And load CCodec as a plugin.

Bug: 74403547
Test: adb shell am instrument -e size small -w 'android.media.cts/android.support.test.runner.AndroidJUnitRunner'
Test: adb shell setprop debug.stagefright.ccodec yes
Test: adb shell stagefright -S -N c2.google.avc.decoder /sdcard/a.mp4
Change-Id: I42f4a3d8dde0e1ae9fc2dbeab11be3dc3a8576fb