Disable IO on IMMS#mHandler

Now we are ready to lock down IMMS#mHandler to disallow any IO
operation there. At least the original reason why we had to allow IO
on the handler was already addressed [1].

There must be no observable behavior change as long as there is no
assertion failure.

 [1]: I4400519f133638c03205f7a8902f7e00c291eda8
      f4887903

Bug: 343601565
Fix: 196206770
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Iba30c7706a991ec5a0a8b94bb62e47a25e6cb4e0