Tweak the conditions to hide IME switcher icon.

With this CL, the visibility of IME switcher is basically determined by
the number of enabled subtypes except for the following veto rules:

  Rule A:  Hide IME switcher if
    A.1 There is a physical keyboard attached, and
    A.2 The current IME is not receiving physical key events.

  Rule B:  Hide IME switcher if
    B.1 There is no physical keyboard attached, and
    B.2 The current IME does not have any visible UI element.

Bug: 25432652
Change-Id: I6d818f5a698b74708253be7a5350621f99a8f703
1 file changed