Introduce new window insets launch flag

Instead of turning off the new window insets system fully on or
off, we introduce three modes:

0: Old system
1: New system for IME only
2: New system for IME + system bars

This allows us to launch the feature in IME only mode, in order to
avoid a lot of compatibility fallout around system bars.

Test: adb shell setprop persist.wm.new_insets 0/1/2
Bug: 118118435
Change-Id: Iaa49e62930b2539770cd313567dde4b102216097