aflags: list flags as "enabled|disabled" instead of raw bool value

To keep things consistent and to reduce the risk of confusion, aconfig
flags are "enabled" or "disabled", not "true" or "false".

Do not represent flag states via a String. Instead, introduce a new
FlagValue enum and use this enum instead. FlagValue implements ToString
which returns "enabled" or "disabled".

Bug: N/A
Test: adb shell aflags list # verify that flags are "enabled" or "disabled"
Test: atest aflags.test
Change-Id: I9be998d7891656d118b7cc971449da58e17d5031