Add mainline_sdk device and product

This device/product is suitable for a soong-only build that builds for
all the architectures needed for mainline modules sdk prebuilts.

Bug: 174315599
Test: TARGET_PRODUCT=mainline_sdk soong_ui --make-mode --skip-kati;
      (inspect soong.variables)
Change-Id: I3ab5d3611e25a666de39752dedaddbf32ddf6df7