Devices that have support for DISPLAY_DECORATION layers may also have
support for a blockingRegion, which will allow the DPU to skip regions
that are known to be transparent, in order to save power.

Use the transparentRegionHint to compute the blockingRegion. It may not
be accurate for all layers, which is why it is only used for
DISPLAY_DECORATION layers.

Add tests.

Bug: 212736475
Test: libcompositionengine_test
Change-Id: I6c5d29614dc8d65d8481150ce717a440192083a0