[automerge] Canonicalise path before extracting relative path 2p: 43b718413e

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/providers/MediaProvider/+/20942347

Bug: 228833816
Bug: 228450832
Change-Id: I22f4cc655d9b4d1e96ca382b3237b333b78c0aad
Merged-In: Id620644ffdfe20e9281773e2e23851c56732dd11