meminfo: handle multiple buffer references in dmabufinfo

Change the dmabufinfo library to handle multiple references to the same
buffer across PIDs by doing these changes:

- Add function AppendDmaBufInfo: appends the information for a specific
PID to an existing DmaBuffer vector
- Change the way fd/map references to a buffer belonging to a PID are
stored in DmaBuffer from vector<pid_t> to unorderedmap<pid_t, int>
- When parsing proc/%pid/{maps|fds}, look up the buffer by inode and
update the data if found

Test: builds
Bug: 63860998

Change-Id: If22feea772f182086ae3446b46d626a750674b1f
Signed-off-by: Erick Reyes <erickreyes@google.com>