Use focused app's stack for IME adjustment

Don't use mInputMethodTarget as it could be set to some system alert
window that has both FLAG_NOT_FOCUSABLE and FLAG_ALT_FOCUSABLE_IM,
and it's not actually taking inputs.

bug: 28328085
Change-Id: I38263f64fec13a5e10357499ebc97671389ba69a
2 files changed