aconfig: move create flag info file api to aconfig_storage_write_api

rust_ffi_static right now will export excessive symbols, this lead to a
duplication of symbols when aconfig links against both
libaconfig_storage_file_cc and libaconfig_storage_read_api_cc. This is
because aconfig_storage_read_api crate depends on aconfig_storage_file
crate. To solve this problem, move create flag info file api to
aconfig_storage_write_api crate which is parallel to
aconfig_storage_read_api crate.

Bug: b/321077378
Test: atest aconfig_storage_file.test; atest
aconfig_storage_read_api.test; atest aconfig_storage_write_api.test

Change-Id: Ibbb50193a2da82d52ccbb4087c8e3fb9f320805f