floss: Add a libflags shim

The code generated by aconfig depends on libflags.
To be able to use aconfig on floss we add a shim version of
libflags.

Bug: 307006275
Flag: EXEMPT, floss
Test: N/A
Change-Id: I7bcc42a0bd4fc86b74fd83a7101d1a82bb6c62dc