Inline ImeTracing#onShellCommand()

This is a preparation to add new command:

  adb shell cmd ime tracing save-for-bugreport

which is to be an important piece of functionality to get rid of an
unintentional behavior change (Bug 177462676).

Anyway this CL itself is still a mechanical refactoring hence there
should be no observable behavior change.

Bug: 177462676
Test: "adb shell cmd input_method tracing start" works.
Test: "adb shell cmd input_method tracing stop" works.
Test: "adb shell ime tracing start" still works.
Test: "adb shell ime tracing stop" still works.
Test: "adb shell ime list" still works.
Change-Id: I972f287bddda962dd42d4dd5226cb8b42e759121