aconfig: use an enum to represetn flag type

Flag type is encoded as a u16 in flag table file. Previously we are
assinging value of 1 for flag type to indicate that they are all boolean
flags. In this change, we formalize the flag type designation with an
enum.

ReadWriteBoolean -> 0
ReadOnlyBoolean -> 1
FixedReadOnlyBoolean -> 2

Bug: b/321077378
Test: atest aconfg.test; atest aconfig_storage_file.test
Change-Id: I114818435b44f1e82a611f0509787993a5a8e70e