Fix permission check bypass in IMMS#dump()

This is a follow up CL to our previous CL [1], which enabled IME
tracing result to be automatically included into the bugreport.

In the previous CL, we checked android.Manifest.permission.DUMP in
IMMS#doDump() not in IMMS#dump(), which means there was a chance that
some priviledged operations could be triggered by a user-mode
application.  So far the only actual imprecation is that a user-mode
process can restart an on-going IME trace session, but in general such
an operation should be allowed only to processes that have
android.Manifest.permission.DUMP or shell access.

With this CL, all the operations inside IMMS#dump() will require
android.Manifest.permission.DUMP again as we did before the previous
CL was submitted.

Other than that, there should be no behavior change in this CL.

This is a preparation to fix another unintentional behavior change
(Bug 177462676), which was also accidentally introduced in the same
CL.

 [1]: Ie87eb8423e2bb70f28c330983d45b95e2e07062d
      ac24994a

Bug: 154348613
Bug: 167948910
Bug: 177462676
Test: Manually done as follows.
  1. adb shell ime tracing start
  2. adb logcat -s imeTracing:*
       Make sure IME tracing started.
  3. adb shell am start -a android.intent.action.DIAL
  4. adb bugreport bugreport.zip
  5. adb logcat -s imeTracing:*
       Make sure IME tracing is stopped then restarted.
  6. unzip -v bugreport.zip | grep ime_trace_clients.pb
       Make sure "ime_trace_clients.pb" is included.
Change-Id: I3de965bc2993df67b046f3e6c09ece11047d227e