Use @EnforcePermission for IIMM#addVirtualStylusIdForTestSession()

This CL manually migrates

  IInputMethodManager#addVirtualStylusIdForTestSession(),

which was added recently [1], from manual permission check with

  Context#enforceCallingPermission(INJECT_EVENTS)

to code-generation check with

  @EnforcePermission("INJECT_EVENTS").

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

 [1]: I7399c0a56c04f024d65c16d459818cc2c5cdbc7f
      d96ef57b

Bug: 34886274
Bug: 210039666
Bug: 232058525
Bug: 237316307
Test: atest CtsInputMethodTestCases:InputMethodManagerTest#testShowInputMethodPickerFromSystemProtection
Change-Id: Ida227d417496dd91c071c38ca0be003529764de4