arm: Support qemu-android on generic arm targets

When building a generic arm 32-bit target, we also want to include
support for the ranchu board model for the updated Android emulator
based on recent upstream QEMU.

Since the emulator.mk file is included by both the generic and
generic_arm64 targets and already defines a PRODUCT_COPY_FILES and
PRODUCT_PACKAGES, move duplicate entries from
target/board/generic*/device.mk to target/product/emulator.mk.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Change-Id: I7922ec0c4097776a185dbb245301d760ff332386