Protect isInputMethodPickerShown() with TEST_INPUT_METHOD permission

IInputMethodManager#isInputMethodPickerShownForTest() was introduced
in Android P (API 28) to verify IME picker visibility in CTS [1].

To make it clear that that IPC method must be available only for
special testing purpose, this CL introduces an @hide permission

   android.permission.TEST_INPUT_METHOD

and requires it in

   InputMethodManagerService#isInputMethodPickerShownForTest().

This CL grants that permission to the shell process hence CTS tests
can still access to the corresponding test API by using
UiAutomation#adoptShellPermissionIdentity().

 [1]: I4e21625c32a0ca1abc740229efb3c7fcd97141cc
      eb570618

Bug: 237317525
Test: atest CtsInputMethodTestCases
Test: Manually verified as follows.
 1. adb logcat -b events | grep 237317525
 2. atest CtsInputMethodTestCases:InputMethodManagerTest#testIsInputMethodPickerShownProtection
Ignore-AOSP-First: For a security fix
Change-Id: Ie79a3e9d41ce22605ae083594d639c37d08b7def