Optimized workflow for IME tracing on InputMethodService side

Optimized the tracing logic for the IMS information. The
InputMethodService triggers a tracing dump through the new method
triggerServiceDump, exposed by the meTracing interface. This
change was done to be able to support custom dumps from clients
and custom dumps from InputMethodService.

This change only covers the IMS information. The IMMS
information will be dumped in next changes.

Bug: 154348613
Test: start IME tracing by calling "adb shell ime tracing start"
      end IME tracing by calling "adb shell ime tracing stop"
      pull trace using "adb pull /data/misc/wmtrace/ime_trace_service.pb ime_trace_service.pb"
Change-Id: Icda0f82d76fb7db5b2bd8d021069b1ff15a4e15b