Input method root view should have BOTTOM gravity.

The window had BOTTOM gravity, but the root view was using the
LinearLayout default (START | TOP), meaning an IME with a tall
inputArea could effectively push the navigation bar guard view
off screen.

This change simply uses BOTTOM instead of TOP for the root view.

Bug:11237795
Change-Id: I681c2b1eddac53589e0d168f09a183d67b1e5322