Canonicalise path before extracting relative path

This helps us make accurate access checks on the given path.

Bug: 228833816
Bug: 228450832
Test: atest FileUtilsTest
Test: atest LegacyStorageHostTest
Change-Id: Id620644ffdfe20e9281773e2e23851c56732dd11
Merged-In: Id620644ffdfe20e9281773e2e23851c56732dd11
(cherry picked from commit 93f5186e4b4a044e00a168c55e05fd3835033221)
(cherry picked from commit 798fa4b89e154877e4ddafc3894eb8275a74eb7e)
Merged-In: Id620644ffdfe20e9281773e2e23851c56732dd11
2 files changed