Track update count of IMMS#mMethodMap

InputMethodManagerService#buildInputMethodListLocked() is known be
one of the biggest contributors to CPU and I/O bandwidth consumed
by InputMethodManagerService (IMMS). To check when it gets called
and to see how a CL would affect its call count, it would be useful
if we can easily get the call count of that method.

With this CL, how many times IMMS#mMethodMap was rebuilt will be
included in the dumpsys.

Test: adb shell dumpsys input_method | grep "mMethodMapUpdateCount="
      to make sure the count is increasing.
Bug: 32343335
Change-Id: I05850578cbf2e6adfea6c7e9b1fb9ec4e9d5e2c9