Make it possible to change libc++ library files used for android
Make it possible to change libc++ library files used for android

Useful for specifying specific or modified libc++ library files instead of
always using the one in android_ndk_root.

Review URL: https://codereview.chromium.org/1215713006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#337797}

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