Add libc_headers header lib

This commit extracts `libc_headers` for `libasync_safe` and
`libpropertyinfoparser` (in the `system/core` repository).

Before this change, `libasync_safe` expects that `libc` is automatically
added to `system_shared_libs` of the libasync_safe vendor variant even
if `libc_defaults` explicitly declines any `system_shared_libs`.

This commit defines `libc_headers` for `libasync_safe` and
`libpropertyinfoparser` so that they can find the headers from libc
without causing circular dependencies.

Bug: 123006819
Test: make checkbuild
Change-Id: I2435ab61d36ff79ca2b4ef70bd898b795159c725