Merge SoftInputWindow#initDockWindow() into IMS#onCreate()

This CL merges

  SoftInputWindow#initDockWindow()

into

  InputMethodService#onCreate()

so that we can see what parameters are set to the IME window at a
glance, rather than having to check 2 different files.

The end result is expected to be the same, and in theory there should
be no observable behavior change.

If you are reading this commit message to look for why those
parameters have been set, here are some quick links to relevant CLs.

 * `FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS`
  - DecorView#mNavigationGuard is gone [1]
 * `window.setDecorFitsSystemWindows(false)`
  - Updated InputMethodService to not inset by navigation bar if
    requested by automotive. [2]
  - Smooth out IME animation for automotive devices [3]
 * `setFitInsetsTypes(statusBars() | navigationBars())`
  - Do let IME fit invisible insets [4]
 * `setFitInsetsSides(Side.all() & ~Side.BOTTOM)`
  - Make IME fit navgation bars at left and right sides [5]
 * `receiveInsetsIgnoringZOrder = true`
  - Let IME receive insets ignoring z-order [6]

 [1]: I664630099b6eb3fe31675444ba94944cb0eb98b0
      8f162c6e
 [2]: I4faf82bdd7536bd2d049ded04034a9635d8ca0d3
      7eec316f
 [3]: I5d7b03d5c829a2679efdd06fa961d1158494e08f
      b0d0d7c4
 [4]: I6e7d665c55839dfbb14c8d2e5365537416f5f6c6
      145f7118
 [5]: I0ef3d6379a9ae52b3749154d2fdc54e9aa94a9e0
      c8364e38
 [6]: I53c64a5598f246ad577f652156903e4666a30cd9
      ea491da8

Bug: 192412909
Test: presubmit
Change-Id: If0f2bac45e6752612cc11e57d2fac55626221bd1