Add talkback hint to Wi-Fi hotspot preferences

- Set the title as the edit box hint.

Bug: 385857484
Flag: EXEMPT bugfix
Test: Manual testing
atest -c com.android.settings.wifi.tether.WifiTetherPasswordPreferenceControllerTest \
         WifiTetherSSIDPreferenceControllerTest

Change-Id: Ie7fe1c29d5f45ce47c7f393bf433ed3b2bcacb59