Remove deprecated hidden public methods from InputMethodUtils.

This is a follow up CL for a recent attempt to minimize
the number of default enabled IMEs.
- part1: I831502db502f4073c9c2f50ce7705a4e45e2e1e3
- part2: Ife93d909fb8a24471c425c903e2b7048826e17a3
- part3: I6571d464a46453934f0a8f5e79018a67a9a3c845
- part4: I871ccda787eb0f1099ba3574356c1da4b33681f3

This CL removes following deprecated hidden public methods
from InputMethodUtils as planned.
- isSystemImeThatHasEnglishKeyboardSubtype(InputMethodInfo)
- isValidSystemDefaultIme(boolean, InputMethodInfo, Context)
- containsSubtypeOf(InputMethodInfo, String, String)

This is a pure code refactoring with preserving the current
logic. Hence no behavior change is intended.

Change-Id: I1ff994cbbdef83e1e907a0d88aa9ae09d45263b4