Tell HWComposer the dimensions of virtual displays

HWComposer queries the HWC for dimensions of physical displays, but
can't do that for virtual displays. The dimensions are used to set the
display frame of the framebuffer target layer passed to HWC, and
implicitly the dimensions of the virtual display.

Bug: 8316155
Change-Id: I9cbd2530d2fa878f86128a1472def520b5d694a5