Use @EnforcePermission for IIMM#showInputMethodPickerFromSystem()

This CL manually migrates

  IInputMethodManager#showInputMethodPickerFromSystem(boolean, int)

from manual permission check with

  Context#checkCallingPermission(WRITE_SECURE_SETTINGS)

to code-generation check with

  @EnforcePermission("WRITE_SECURE_SETTINGS").

There should be no observable semantic behavior change in this CL.

Bug: 34886274
Bug: 232058525
Bug: 237316307
Test: manually verified that SecurityException is thrown
Change-Id: I9d1be1aa65276fcc50c2868fed6b0447d54ff7ee