sdk-addon: Prevent nested data/ inside $(PRIVATE_STAGING_DIR)

Current logic recursively copies $(PRODUCT_OUT)/data into $(PRIVATE_STAGING_DIR)/data
This leads to creating $(PRIVATE_STAGING_DIR)/data/data on incremental
builds causing emulator to fail to boot.

Test: Build emulator sdk_addon twice, extract the zip, run the emulator
and verify it boots and reaches homescreen

Change-Id: I9e74d2ab28f9fbd67362578672d25d438fad0d53