Adds system-as-root VTS test

All new devices launched in P must use system-as-root.
Kernel needs to support mounting system.img with dm-verity as root.
BOARD_BUILD_SYSTEM_ROOT_IMAGE must be set to True as well.

Bug: 64194858
Test: vts-tradefed run commandAndExit vts -m VtsKernelSystemAsRoot
Change-Id: I05bf5116ba7679d7f8d0003b2730b4ef382be01b
4 files changed