Check /mnt/expand paths as if /data.

The layout of adopted storage devices is identical to the internal
userdata partition, so we should re-use the same logic to determine
if a given path is legitimate.

Bug: 77654971
Test: adb shell /data/nativetest64/installd_utils_test/installd_utils_test
Change-Id: I856788a8e9910f618a4f259a20a264e14b46e079