Add a utility method to InputMethodInfo for FBE.

This is another preparation for Bug 26279466.  Whether an input method
is marked to be encryption-aware or not would become the thing here and
there in the Framework code.  Having a utility method and debug info
in dumpsys should make our lives easier.

Bug: 26279466
Change-Id: Icf921fe3661eccf4a589b08b616d05decc561356