Unify camera2-pipe metadata interfaces with camera-common

This CL migrates CameraPipe to use androidx.camera.common.Metadata
while maintaining a backward-compatibility layer for downstream
Google3 consumers:
- Migrate internal CameraPipe implementations to use common.Metadata.
- Provide a deprecated pipe.Metadata interface extending common.Metadata
  with default delegation for legacy lookups.
- Provide a deprecated pipe.Metadata.Key class and MetadataKey typealias.
- Make FrameMetadata and RequestMetadata extend pipe.Metadata.

Change-Id: I7b51bb12f6c5225419fb6e978642228a43dff1be