Fix external manual calculation bug.

Ignore file extensions anywhere under "Android" since those files
belong to specific apps.  Don't depend on GID being set for those
directories.

Bug: 35684969, 35441006, 34945234
Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Change-Id: Ifeb8d314cc2b803ac0d3595974daf91f51862061