Don't use implementation-defined format with CPU consumers

If the virtual display surface is being consumed by the CPU, it can't
be allowed with HAL_PIXEL_FORMAT_IMPLEMENTATION_DEFINED since there is
no way for the CPU consumer to find out what format gralloc chose. So
for CPU-consumer surfaces, just use the BufferQueue's default format,
which can be set by the consumer.

A better but more invasive change would be to let the consumer require
a certain format (or set of formats?), and disallow the producer from
requesting a different format.

Bug: 11479817
Change-Id: I5b20ee6ac1146550e8799b806e14661d279670c0