Add conversion functions.

There are 5 pairs of wrapper classes for the 5 HIDL interfaces: IOmx,
IOmxNode, IOmxObserver, IOmxBufferSource and IGraphicBufferSource.
Some additional functions of OMXBuffer and OMXFenceParcelable are needed
in the implementation of these wrapper classes, so they are added in
this CL as well.

Auto-generated files that correspond to interfaces without queryable
instances are removed.

Test: Compiles.

Bug: 31399200
Change-Id: Iaec9c0e0afee6fc847f324c3eb3c105d6342a1fe