Use Context#getSystemService(Class<T>) in IMMS.

This is a mechanical replacement of
  Object Context#getSystemService(String)
with
  T Context#getSystemService(Class<T>)
in InputMethodManagerService.java.

No behavior change is intended.

Bug: 26279466
Change-Id: Iec6b89f0367140f98d3ca4caa1dae7375e27f3ad