Adding boot-test-harness.img

In commit I30137c3caef91805d9143d404e5e4d06c0fccc30, we added
a boot-debug.img to allow adb root when using an user build GSI image.

However, to run automated tests, it requires additional properties,
which are not needed for GSI compliance:
    ro.audio.silent=1
    ro.test_harness=1

This CL adds an additional boot-test-harness.img for automated tests,
and keeps the original boot-debug.img for GSI compliance.

Note: boot-test-harness.img won't be built by default, it needs
      explicit `make bootimage_test_harness`.

Bug: 140036184
Test: `m bootimage_test_harness`, flashes boot-test-harness.img and checks
      adb root works and test harness props are set.
Test: `m bootimage_test_harness dist -j32`, checks both
      boot-test-harness.img and ramdisk-test-harness.img are under ./out/dist/.
Test: `system/tools/mkbootimg/unpack_bootimg.py --boot_img $OUT/boot-test-harness.img --out ramdisk-test-harness`,
      checks the extracted out/ramdisk is as expected
Test: Run `gunzip -c ramdisk | cpio -idm` for the ramdisk extracted from
      $OUT/boot-test-harness.img and $OUT/boot-debug.img, respectively.
      Then compare the root dirs of both, e.g.,
      `diff -rq --no-dereference ./ramdisk-test-harness ./ramdisk-debug`
Test: `m ramdisk_test_harness-nodeps` and `m bootimage_test_harness-nodeps`
Change-Id: Iadea0b5c933c3b7fa10dcf3d9e85596916b3333d