NDK sysroot: include each license file only once

bionic/libc/NOTICE file is 6000 lines long and is referenced by 10
modules in bionic/libc/Android.bp. We're currently including 10 copies of
the license file, which makes it hard to view a "platform prebuilts" CL
in Gerrit.

For example, click "Expand All" on:
https://android-review.googlesource.com/c/platform/prebuilts/ndk/+/730825

Bug: none
Test: m out/soong/ndk.timestamp
Change-Id: I857e70d7d900688532ab988800c98bbd0899cc31