aflags: show staged values for each flag.

Lines now have a new column, and look like this:

com.android.example.flag1 enabled -            server read-write system
com.android.example.flag2 enabled (->disabled) server read-write system

The dash represents no change on boot, the (->value) represents a change
to value on the next boot.

Test: adb shell device_config put staged multitasking*com.android.wm.shell.enable_taskbar_navbar_unification true && adb shell aflags list | grep navbar_unification
Bug: 324436145
Change-Id: I022460bc69fcb2ccd9c6db8f060fcbd0337d1ea6