Fix testVerifyStatsExternalConsistent case

Root cause: For devices mount /data partition without quota option will
get this testcase failure, because
the getExternalSize logic will use Android/obb path
to calculate totalsize.
But the testcase will check the totalsize excluding obb.

Solution： For device mounted userdata without quota option
will remove obbSize from totalSize, to
Make android.appsecurity.cts.StorageHostTest
testcase PASS.

Test:  run cts-on-gsi -m CtsAppSecurityHostTestCases -t
android.appsecurity.cts.StorageHostTest

Bug: 145269983
Change-Id: I8df02d5427d7512046ca3793f03a133a4ad00cbd