Add relative_install_path property to prebuilt_etc

This supports a more consistent property across modules for specifying a
subdirectory to install a file into for prebuilt_etc modules.

Updates bpfix to rewrite `sub_dir` to `relative_install_path`.

Test: gotest prebuilt_etc_test
Test: gotest bpfix_test
Bug: 156568187
Change-Id: Idd05cd2178c46e290764a3b708faa8275818ca1e