Allow read/write aconfig flags to be read before boot complete

We should now be able to access R/W aconfig flags immediately during
boot, thanks to a new storage mechanism. See bug for details and links.

Bug: b/380147997
Test: manual testing of new R/W flag and validation of dumped flags
Flag: EXEMPT N/A
Change-Id: Idc77e2d767c6839b11776372f101af75e4410d61