Use implementation-defined format when HWC writes to output buffer

When GLES isn't writing to the output buffer directly, request an
implementation-defined format with minimal usage flags, leaving the
format choice up to gralloc. On some hardware this allows HWC to do
format conversions during composition that would otherwise need to be
done (with worse power and/or performance) by the consumer.

Bug: 8316155
Change-Id: Iee6ee8404282036f9fd1833067cfe11dbadbf0bf