Add recovery image sdk trait to cc_library_headers

Allows an sdk to require that a cc_library_headers module provides a
recovery image variant for the prebuilt.

Previously, "recovery_available: true" would be set in the generated
prebuilt snapshot for any sdk member that specified
"recovery_available: true" in the source module. This change will only
add that setting to the snapshot if the recovery image variant trait
was explicitly requested for a member.

Bug: 195754365
Test: m nothing
Change-Id: I7d79ccdec843127f7852d82b4b163021e30a79a7