Capture snapshot headers in parallel

VNDK and vendor snapshot singleton work in a single thread, so globbing
in singleton results in ridiculus running time. Moving codes to
GenerateAndroidBuildActions to reduce running time.

Bug: 150406226
Test: VNDK_SNAPSHOT_BUILD_ARTIFACTS=true m dist vndk vendor-snapshot
Test: vendorSnapshotSingleton build time became 0.56s (from 10s)
Test: build.ninja building time became 1m11s (from 1m21s)
Change-Id: I4a081eef5847c62ca00280ca426f5b4e10f87b59