Add assumption for test to execute only when feature flag is on

Add assumption to make sure that the test did not fail due to errors in
turning the feature flag on.
This is done as the feature flag will be turned on soon (tentatively by
end of week).

Bug: 173648980
Test: atest ExternalStorageHostTest
Change-Id: Icb0082220276905025dc4d9441d5ae9f3a620a87
2 files changed