Fix version script paths on Soong
Fix version script paths on Soong

This commit updates `library.go` to avoid using
`getBackendPathInSourceDir` when computing version script paths
on the Android.bp backend as Soong adds the prefix automatically.

Signed-off-by: Alexander Khabarov <410973c8966e1f78eb22e25cb3609e3bbce9769a@arm.com>
Change-Id: Ifa7428bef6056fd4c25fa86b49cc108560007f16

ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build