Introduce IME Subtype getLayoutDisplayName API

Following the existing IME Subtype getDisplayName API, this new method
will return the name of the IME Subtype's layout as specified by the IME.

When available, this will be displayed on a separate line in the IME
Switcher Menu, under the IME Subtype's display name.

Flag: android.view.inputmethod.ime_switcher_revamp_api
Bug: 371168848
Test: atest
  InputMethodInfoTest#testInputMethodSubtypeWriteToParcel
  InputMethodSubtypeTest#testLayoutLabelNonLocalizedParcel
  InputMethodSubtypeTest#testNoLayoutLabelNonLocalizedParcel
  InputMethodSubtypeTest#testSetLayoutLabelNonLocalized
  InputMethodSubtypeTest#testSetLayoutLabelNonLocalizedWithLayoutLabelResource
Change-Id: I39a82cf66f0890a671df35f3c73adcdf88959617