Update check_target_files_vintf to check the last line of stdout.

More output is now produced on stdout by checkvintf.
Update logic to compare against the last line for incompatible.

Fixes the following tests:
test_check_target_files_vintf.CheckTargetFilesVintfTest#test_CheckVintf_kernel_incompat
test_check_target_files_vintf.CheckTargetFilesVintfTest#test_CheckVintf_matrix_incompat
test_check_target_files_vintf.CheckTargetFilesVintfTest#test_CheckVintf_sku_incompat

Bug: 251221029
Test: atest releasetools_test
Change-Id: I592acf9a3868e2a60e203b2201e7fadcbb0e7f04