Avoid copying of font table data, provide raw font bytes

Minikin is changing its approach to table access to use HarfBuzz to
access the tables, based on raw font data, rather than calling the
MinikinFont::GetTable() virtual method. This patch provides raw access
to the font data to make this work.

There's a bit of plumbing to make sure fonts get a pointer to the raw
data as well.

Bug: 27860101
Change-Id: I638e18cf363644bf22fbc9fb9b3358a9e731087f