Add flag_storage_backend field to aconfig declaration and parsed_flag

The flag_storage_backend is an enum that assumes four possible values:
UNSPECIFIED / ACONFIGD / DEVICE_CONFIG / NONE. The default value will be
UNSPECIFIED if it is left unset in aconfig declaration.

Developers should not explicitly select a storage option. The proper storage option will be determined by aconfig binary while parsing the flag declaration and value files.

Mainline beta RW flag will be assigned DEVICE_CONFIG, all RO flag will be assigned NONE, all other
regular RW flag ACONFIGD.

Test: cd build/tools/aconfig; atest -c
Bug: 406818480
Change-Id: I9b164ec18b74f6f905bf921e7c6c3a274d13bf2f