Let insets source be user-controllable even if it doesn't provide insets

If an insets source doesn't provide insets, the app is not able to
change its position. But the app should still be able to change the
alpha.

Fix: 285584763
Test: WindowInsetsTests:
      Drag to show floating IME in 3-button nav mode in landscape.
      See if the visibility of IME is as expected.
Change-Id: I3210df9bcc087f5fba712e0e7ac4922475c3b88d