Move emulator.mk out of full_base.mk

Instead we should have the emulator products (full, full_mips, full_x86,
sdk) inherit from emulator.mk directly, because full_base.mk is also
inherited by real device products, such as full_manta.mk.

Conflicts:
	target/product/full_base.mk

Change-Id: I6f5f9255c73be8a4cfad7ddf88c4b633b05a7091