Add a property in ndk_library for header contributions

The new property will be used to determine the header file contributions of
ndk_library(s) to the Public API surface. This should be a no-op for
regular Soong builds.

This will be used by a future bp2build converter to populate
the BUILD files for Multi-tree `cc_api_contribution` targets

(Also noticed that sdk_test.go was never added to testSrcs, which this
CL should fix)

Test: go test ./cc
Test: TH

Change-Id: Ieea093e4aac68e341c6414b6cafe02c441643cdf