Add sleep times in ScopedStorageDeviceTest tests

Added sleep times between create and delete operations in
ScopedStorageDeviceTest to reduce flakiness if the dentry cache is still
not updated.

Test: atest ScopedStorageDeviceTest
Flag: EXEMPT test fix
Bug: 360100744
Bug: 332790706
Change-Id: I21e121b496f842eff564ca9c8b92bede61d6582c