[Wi-Fi] Fix crash after editing invalid hotspot password

From Android 11, Wi-Fi framework only supports ASCII encoding
for hotspot password, APP should check if password character is
valid.

This change checks if SoftApConfiguration accepts the password
and disable the OK button if the password is invalid.

Bug: 163353576
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiUtilsTest
Change-Id: Ief3c6c1f08f8fcdf128768cde3ab8eef91f19fbe