graphics: support FB HAL using HWC2OnFbAdapter

FB (framebuffer) HAL has been replaced by HWC HAL for 5+ years, but
we still support the legacy path in SurfaceFlinger.  Devices using
the legacy path cannot be Treblized.

This change allows such devices to use HIDL IComposer, by adding
support for FB HAL in the default implementation.

Test: boots hikey960
Change-Id: Ie9050bbcaac0fd5b134786f4f9f0f5075f4ebd0c