Introduce InputMethodInfoSafeList behind the flag

With this CL we start using InputMethodInfoSafeList to transfer
List<InputMethodInfo> over Binder IPC so that we do not need to worry
about TransactionTooLargeException any more. The new behavior is fully
flag-guarded.

This should enable us to remove our past workarounds [1][2] in the
future.

 [1]: Ibb2940fcc02f3b3b51ba6bbe127d646fd7de7c45
      f0656956
 [2]: I51703563414192ee778f30ab57390da1c1a5ded5
      eed10082

Bug: 339761278
Flag: android.view.inputmethod.use_input_method_info_safe_list
Test: atest FrameworksInputMethodSystemServerTests:InputMethodInfoSafeListTest
Change-Id: I0a7667070fcdf17d34b248a5988c38064588718a