VSDK: use relative install path when generating snapshots

Without respecting the relative install path, the snapshot install
path may collide when they use the same stem name.
To avoid this, respect the relative install path when generating the
snapshots.

Bug: 279652606
Test: RECOVERY_SNAPSHOT_VERSION=current m recovery-snapshot
Merged-In: Ic70a5855aeb2a6eda397b4dd06113dddb6ef70f4
Change-Id: Ic70a5855aeb2a6eda397b4dd06113dddb6ef70f4
(cherry picked from commit 17d0ee21)