BLASTBufferQueue: Pass same default usage as BufferQueueLayer

The default usage value for BufferQueueLayer is COMPOSER not
FRAMEBUFFER (see Layer::getEffectiveUsage). It's unclear exactly
what effect this was having.

Bug: 152501005
Test: Existing tests pass.
Change-Id: I33adc9c39c57b11c71a53e936448c9052bd83155