Revert "Allow IMEs to use light nav bar buttons"

This reverts commit af2e20e1 [1].

Reason of revert:
That commit causes an unexpected behavior change (Bug 69111208)
when the target application is using light navigation bar and
the IME does not override the navigation bar background color.

 [1]: I2cecadb82d14564d7019a9636918b06c3bcf433c

Bug: 69002467
Bug: 69002565
Fixes: 69111208
Test: Manually verified as follows:
  1. Make sure the current IME does not override navigation bar
     background color. (e.g. AOSP keyboard and SoftKeyboard sample)
  2. Open any app that specifies light nav bar (e.g. Settings app)
  3. Make sure that the navigation bar background is not dark and
     navigation bar buttons are rendered with dark color.
  4. Focus any EditText on that app.
  5. Make sure that Navigation bar background color becomes
     R.color.input_method_navigation_guard, which is black on AOSP.
     Also navigation bar buttons are rendered with light color.

Change-Id: I5e2e3007419f0ee81383b115f4073d5831c5d890