[flexiglass] Allow dual notification shade to align to the bottom.

This allows NotificationsShadeScene to be customized such that instead
of align to the top, it is now align to the bottom (positioning to the
right side remains unchanged).

This also adds a new transition to the framework so that when it is on
the bottom, the scene appears from the bottom edge rather than the top.

Flag: com.android.systemui.scene_container
Bug: 343744476
Test: Manual; Change config.xml content to be true instead, and then
call on adb shell service call statusbar 1 to see animation
Test: Manual; verify on Goldfish phone emulator with flexiglass on flag
off still works

Change-Id: Id44ad8ca9be2a9d2b35caed657589eebe164160c