fall back to libdvm.so if the custom runtime lib does not exist

when providing builds without ART it can happen, that the user
gets a bootloop if he set to use ART before and then flashes the
build without ART

also harden against failures, if the user manually sets the runtime
and does a typo.

resolution to this problem, check if the custom runtime library exists
and if not, fall back to dalvik libdvm.so

Change-Id: I0b944f1a3b63893ab58af7355a226ae2b9c62fe4