Introduce a runtime flag for InputMethodInfoSafeList

In order to address TransactionTooLargeException when trying to pass
List<InputMethodInfo> over Binder IPC, we are planning to introduce

  InputMethodInfoSafeList,

which relies on Parcel#{read,write}Blob(). As a preparation this CL
introduces a new runtime flag.

There must be no observable behavior change (yet).

Bug: 339761278
Flag: android.view.inputmethod.use_input_method_info_safe_list
Test: presubmit
Change-Id: Id74a17fe7e3655177af63894dde92772cf26a5f2