emulator: factor vendor partition out of emulator.mk

The emulator vendor paritions are better moved out of
emulator.mk into device/generic/goldfish/vendor.mk
so that changes in vendor.mk will just concentrate on
vendor partition.

this cl does not impact real devices, and it does not
even impact emulator images (it does code refactoring only)

BUG: 110030159

Change-Id: Ic5778cda8ca6ccf7bc2bc2f9028f801cda970143