Add a metadata field to BufferDesc

This change adds a variable-length metadata field to BufferDesc.  This
new data is owned by EVS Camera HAL implementations and opaque to EVS
manager.

Change-Id: If9b25093afa55ea0782af776a3f67923664e0058
Fix: 142891952
Bug: 142888671
Test: Build successful
Signed-off-by: Changyeon Jo <changyeon@google.com>