Stop supporting null IME token in IMM#switchToLastInputMethod()

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#switchToLastInputMethod().

Bug: 114488811
Test: CtsInputMethodServiceHostTestCases
Change-Id: Icb02c9bb52b11cff39b222198f4b67984676b9a6