Preset the test environment for WifiSliceTest

Currently WifiSliceTest needs a manual setup step before testing, which
is not a good practice for the automated test suite.

Check if the DUT has a default assistant app, if so, then check if it's
granted the slice permission; if not, just auto-set a default one in
SettingsProvider and then check that one. So that we can pass the test
without the manual setup step.

Test: atest WifiSliceTest#wifiSlice_grantedPermissionToDefaultAssistant
Bug: 170512803
Bug: 170326428
Change-Id: I70039bade02fcbc457900c6d958d4a0800fda106