Introduce script and module to pass aconfig flags to metalava

`keep-flagged-apis` module generates a txt file that
contains annotations based on aconfig flags that are passed to metalava
when generating "exportable" or "runtime" stubs.

`keep-flagged-apis.sh` already exists under
`packages/modules/SdkExtensions` directory, but modifying the script
(and the bp module definition) will break the `udc-mainline-prod` branch
in downstream. Therefore, the modified script and the module definition
is defined here, and will be used in the long term.

Test: m keep-flagged-apis && manual
Bug: 315485740
Change-Id: I4ab8d4ea2c6c8ddba6d55b2b25e8c8d3b7c8fa07