Add aconfig flag file

This CL does not have Merged-In since all the branches should have the
aconfig file so that adding flags to the file does not have conflicts.
Following CL adds aconfig_declarations to Android.bp only on branches
where aconfig soong targets are supported

Bug: 267870186
Test: TH
Change-Id: I621228a2c9594bf7bfe1cf335619c417e9339743
2 files changed