aconfig: update flag info query api

Currently flag info query api is called get_boolean_flag_attribute, in
this change, we switched it over to a flag value type generic
implementation get_flag_attribute. So in the future we want to add more flag value types, this api can stay the same.

Bug: b/312444587
Test: atest -c
Change-Id: I2b272f3fa3cb1d0edc8b77a44bf37752ffe95925