Deprecate IMM#showSoftInputUnchecked() part 2

This is the 2nd attempt to have CryptKeeper stop depending on
InputMethodManager#showSoftInputUnchecked() do avoid non-deterministic
behaviors.  Note that the original one [1] was reverted [2] due to
ClassCastException as crypt_keeper_pin_field.xml was not updated to
use ImeAwareEditText.

We have been receiving more and more strange bug reports regarding
the initial IME visibility on CryptKeeper (Bug 31915865, Bug 35198715,
and Bug 36148078).  Most likely we have introduced more bugs before
fixing previous bugs.

As the first step to disentangle multiple problems, this CL addresses
a long standing issue that CryptKeeper relies on @hide internal API
InputMethodManager#showSoftInputUnchecked().  As explained in
Bug 36015425, IMM#showSoftInputUnchecked() is considered harmful in
terms of bypassing standard IME focus handling flows, which prevents
us from taking advantage of historical debugging mechanism that we
have added recently in Bug 35079353.

 [1]: I3cffeec286d9372ff05e85a49b82287cb1add8a2
      4ae46cd1
 [2]: I776ad2e6ba0eabacb3d6be6de23ba37fe5ce7bc7
      cf026ead

Test: Manually verified that Bug 31915865 is still broken but this CL
      does not make it worse.
      1. Flash an image into a non direct-boot device.
      2. Set up a device boot password.
      3. adb reboot
      After the step 3, the software keyboard shows up automatically
      then suddenly disappears (Bug 35198715). Do the same test for
      device boot PIN instead of password.
Bug: 36015425
Change-Id: I182b05d3ff59fb3b4732d60d0d5a464f0e0e0235