Remote mCurClientInKeyguard dependency in IMMS

The above field introduced from CL[1] for quick-fixing unexpected back
icon indicator shows up when the lockscreen is shown.

However, from IMMS points of view, seems IMMS is not necessary to be
the decision maker to tell systemUI whether the IME is behind keyguard
or not.

SystemUI can properly leverage WindowInsets APIs to know the IME insets
state when the keyguard requests IME visible.

so that in other non-keyguard cases should be enough to deliver the
IME visiblity/active information without setting "vis=0" for the
special cases.

[1]: I0de01ec29cb544e902305b0f9d9fb94a73835e7b

Change-Id: I24c511d068ab371d6984201327a561322fc26bd8
Fix: 202218289
Test: manual as steps:
     1) Set passcode for lockscreen to require IME.
     2) Try to unlock screen
     3) See if the back key icon altered when the IME shows up on
        the lockscreen
Test: atest NavigationBarTest#\
        testSetImeWindowStatusWhenKeyguardLockingAndImeInsetsChange