Fix two bugs in external storage accounting.

When doing a manual tree walk, we always want to inherit fts_number
from the parent fts node.

For create_data_media_package_path(), two of the arguments had been
swapped.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Bug: 27948817, 30927649
Change-Id: Ic3cbf387cf119aa82af6e380bde9d9b069f6bfb0