ImageReader: Fix format workarounds for legacy devices

Legacy shim overrides HAL_PIXEL_FORMAT_BLOB to
HAL_PIXEL_FORMAT_RGBA_8888. Make ImageReader aware of the
workaround when returning the format.

Remove the workaround in legacy shim that overrides YUV_420_888
to YV12 because allocating YUV_420_888 buffers with SW access
is supported on Flo.

Bug: 21707937
Change-Id: I84a8610c82ae986a0064873f611e4f257baf1774