Set IME nav bar height when installing unattached

When installing the IME navigation bar height the systemInsets can be
null if the DecorView is not attached to the window yet. In this case we
still add the NavigationBarFrame to the DecorView, with the default
LayoutParams, which resolve the height to MATCH_PARENT. Since ensuring
the IME insets contain at least the IME navigation bar in [1], this
could lead to full size IME insets for a few frames.

This sets the initial IME navigation bar height to the captionBarHeight
when the DecorView is not attached yet.

  [1]: Id252731ac8b5138cccd1164197ff3ece3a5f3cf8

Flag: EXEMPT bugfix
Bug: 401192777
Bug: 387924046
Test: atest WindowInsetsControllerTests#testDispatchApplyWindowInsetsCount_ime
  WindowInsetsControllerTests#testImeInsetsFinalSizeIsMaximumSize
  InputMethodServiceTest#testImeNavigationBarInsets
Change-Id: Icd174e9dd64b33804bedfcc6cdf0ddf90e29fecd