SECURITY: Whitelist DTD declaration in SVG

Only allow ENTITY declarations inside the doctype internal
subset. Do not allow parameter entities, recursive entity
references are entity values longer than 255 bytes, or
external entity references. Filter external doctype subset
to only allow the standard svg doctypes.

Recursive entities that are simple aliases are allowed
because people appear to use them on commons. Declaring
xmlns:xlink to have a #FIXED value to the xlink namespace
is allowed because GraphViz apparently does that so its
somewhat common.

This prevents someone bypassing filter by using default
attribute values in internal dtd subset. No browser loads
the external dtd subset that I could find, but whitelist
just to be safe anyways.

Issue reported by Cassiogomes11.

Bug: T151735
Change-Id: I7cb4690f759ad97e70e06e560978b6207d84c446