Extract top level directory for public volumes

Changed extractTopLevelDir to match public volume paths and extract top
level default directory. Added new test for extractTopLevelDir

Test: atest FileUtilsTest#testExtractTopLevelDir
Test: atest packages/providers/MediaProvider
Bug: 148841336
Change-Id: Id1293bcbde078c96a062f2a72a193ae93c52fc9f
2 files changed