Added native metadata support.

Metadata.java:
Fixed typo 8k != 8092. The comment was correct though.

In Metadata.h, the new Metadata class is declared in the ns android::media
to limit the chances of conflict with other packages.

The MetadataType in MediaPlayerInterface is gone and moved to Metadata as
an inner typedef.

Similarly the SortedVector<MetadataType> instance have been replace by a
new type Metadata::Filter.

All the keys declared in the java counterpart are also in Metadata.h.

Metadata.cpp:
Contains the implementation of the native metadata packing.

There an associated change in the opencore package that should go in
at the same time as this one.