Add container field to flag_declarations

A container is software which is always built in its entirety using the
same build environment. In particular, all of its parts are built using
the same build-time default flag values. In addition, containers are
always installed as a single unit.

Bug: 312696545
Test: atest aconfig.test && m all_aconfig_declarations
Change-Id: I2ef3db836c4456f4f4fb5c066edf9094e38f89cc