codec2: fix C2Buffer

- _C2MappingBlock2DImpl::Mapped hold shared_ptr to _C2Block2DImpl
  instead of raw pointer to prevent use-after-free.

Bug: 74073134
Test: adb shell setprop debug.stagefright.ccodec yes
Test: adb shell setprop debug.stagefright.omx_default_rank 1000
Test: adb shell killall mediaserver
Test: atest android.media.cts.MediaCodecTest
Change-Id: Icf3bb6aa781da9916c891a1325480051f6ffdffe