Add plaintext WifiConfig SSID length check

Plaintext WifiConfiguration SSIDs need a length check or else an
arbitrary length may be set. Set the max length to 32 characters, as
the minimum length for any SSID encoding is 1-byte per character.

Bug: 343714914
Test: atest WifiConfigurationUtilTest
Change-Id: I88994f6c022f7b427aceab3fac6cc876a0ecb4c9