installd: Simplify Dalvik cache path creation

Installd computes Dalvik cache path for a dex file by concatenating
'/classes.dex' to the APKs absolute path and then replacing all '/'
characters with '@'. OTA preopt does the same, only in reverse order,
i.e by concatenating '@classes.dex' to altered absolute path.

This patch unifies the two approaches so as to keep only one string
constant.

Change-Id: I69bb6bca831f45c873a0eb8580cf8d4b011f3b09