PTP: Improve performance and reliability of file importing

Now the file copy is done completely within the media process
rather than pushing data to the client via ContProvider.openFile().

File system writes are now interleaved with USB reads, which allows us
to copy the data faster and prevents the camera from timing out during transfer.

File is automatically inserted in the media provider after a successful import
and a Uri is returned to the client.

BUG: 2994234

Change-Id: Ie75c63da76f623343d3d966c6a707aa1ae871972
Signed-off-by: Mike Lockwood <lockwood@android.com>