Refactor BoardConfig.mk for emulator products

The majority of the contents in BoardConfig.mk of emulator products
are the same. The common part is factored out as EmuConfig.mk so it's
easier to maintain these closely related products going forward.

Bug: 80117040
Test: Following products have been built and booted to homescreen:
  $ lunch aosp_x86-userdebug; m -j; emulator
  $ lunch aosp_x86_64-userdebug; m -j; emulator

Change-Id: I5392e0e88865d4908906a67c29939c23af84baf0
Merged-In: I5392e0e88865d4908906a67c29939c23af84baf0
(cherry picked from commit 510b76d8)