Explicitly handle null rather than relying on NPE.

IInputMethodManager is an actual IPC interface with which application
processes can communicate with InputMethodManagerService (IMMS).
Although this has never been a public API, it is actually exposed to the
application process hence we parameters passed to that interface methods
should be considered untrusted.

In Android L-MR1 and prior, calling IInputMethodManager#startInput()
from the application process via reflection with null EditorInfo can
result in NPE in the IME process, which is not great.

In Android M, doing that causes NPE in the IMMS process but the
exception is just sent back to the application process via Binder call,
which is no longer so harmful.

In Android N, we want to make sure that such an invalid internal API
call is clearly under our control, rather than relying on NPE, by having
an explicit null check.

Bug: 26866030
Change-Id: Ica812177d9ca454dd16e3dd6854f3053c329b344