Move trace points to IMMS#onSessionCreated()

This is a follow up CL to our previous CL [1], which introduced
invocations of Trace.trace{Begin,End} into

  InputMethodManagerService.MethodCallback#sessionCreated()

with trace tag:

  "IMMS.sessionCreated".

In order to inline InputMethodManagerService.MethodCallback,

this CL moves these Trace.trace{Begin,End} to

  InputMethodManagerService#onSessionCreated(),

with renaming the tag to

  "IMMS.onSessionCreated".

Other than that, there is no observable behavior change in this CL.

 [1]: I586341426916f9b9e0f1efe988894621972da4ff
      d663b621

Bug: 167947940
Bug: 192412909
Test: presubmit
Change-Id: I4135737825befff9e696f47c153a209de4adb0e3