drm_hwcomposer: refactor platform directory

Motivation:

Platform term meaning used in drm_hwcomposer does not correspond to the
content of the platform directory. Platform directory consists of:
1. Buffer information getters for different gralloc (currently called platform).
2. Composition planner logic (which has flaws and should be reworked into
   layer->plane mapping during validation stage logic).
3. DrmGenericImpoter with reference counting logic.

Android-11 IMapper@4 metadata API offers a generic way to access buffer
information which makes other gralloc buffer information getters obsolete.
Legacy getters should be maintained for some time until all known users
will migrate to Mapper@4 API.

Implementation:

1. Split 'PlatformImporter' logic to 'Importer' only and 'Buffer Getter' logic.
   a. Remove buffer_handle_t parameter from ImportBuffer(). Instead user should
      get BufferInfo using ConvertBoInfo to struct hwc_drm_bo_t, then use it for
      ImportBuffer().
   b. Move DrmGenericImporter.{cpp/h} into the drm directory.

2. Isolate planner code in single file and move it to compositor directory as
   compositor/Planner.{cpp/h}

3. Rename platform definition
   a. Rename platform directory to bufferinfo.
   b. Rename/move bufferinfo/platorm*.{cpp,h} getters to
      bufferinfo/legacy/BufferInfo*.{cpp,h}. Align class names/includes.

4. Split legacy/metadata getters logic.
   a. Apply existing bufferinfogetter base class only for legacy getters.
   b. Combine legacy/generic gettera under new base class.
   c. Create a placeholder for generic(metadata) getter.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
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