Load metadata of LocalData on demand.

1. Metadata of LocalData is not always needed. It's more efficient to load it on
demand.
2. MetadataLoader added to be responsible for loading all the metadata.
3. Metadata loading callback added in LocalDataListener.
4. The loading of the metadata and the bitmap are now all executed by the thread
pool executor.

bug:12216695
Change-Id: I5c75746630e9952b389f9b50416a0bb950301f33
9 files changed