aconfig: add namespace field to flag_declaration and parsed_flag

Add a new field to the proto messages flag_declaration and parsed_flag.

The new field will be used verbatim as a parameter when calling
DeviceConfig.getBoolean to read the value of a READ_WRITE flag. See the
DeviceConfig API for more info.

Note: not to be confused with the old namespace field, which has been
renamed to package.

Bug: 285211724
Test: atest aconfig.test
Change-Id: I2181be7b5e98fc334e5277fb5f7e386f1fe0b550