Stop supporting null IME token in IMM#setInputMethodAndSubtype()

Hopefully no one has relied on this undocumented behavior that when
the caller has WRITE_SECURE_SETTINGS then null IME token is allowed in
IMM#setInputMethodAndSubtype().

Note that if the caller had WRITE_SECURE_SETTINGS they can achieve the
same goal by directly updating the following secure settings:
  * Settings.Secure.DEFAULT_INPUT_METHOD
  * Settings.Secure.SELECTED_INPUT_METHOD_SUBTYPE

Bug: 114488811
Test: CtsInputMethodServiceHostTestCases
Change-Id: Ic5c59299ace16778fc57a4ec2639508564f416a7