Fix KeyboardShortcuts show

The keyboard shortcuts should be shown only if they have not been
dismissed already (by using the dismiss method). Check that sInstance is
not null in the onKeyboardShortcutsReceived callback before displaying
the dialog. Earlier, a call to activity's dismissKeyboardShortcutsHelper
right after a call to requestShowKeyboardShortcuts would not dismiss the
popup which is not the case now.

Test: Manual
Change-Id: I544708b003d6baab92cac2b6fce71698dbbdaf0c