Remove the hard-coded path of build_verity_metadata.py.

build_image.py used to invoke build_verity_metadata.py with a hard-coded
path of 'system/extras/verity/build_verity_metadata.py', which makes it
hard to run unittests from non-$(ANDROID_BUILD_TOP) directory.

This CL adds the dependency on the tool, so that it gets installed to
$(HOST_OUT_EXECUTABLES), then removes the hard-coded path.

Bug: 74544459
Bug: 76015688
Test: `m dist`
Test: python -m unittest test_validate_target_files
Change-Id: I0dcf4eb067a0db6f099cb589eb99a151a05c7f2b