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

cc/ndk_sysroot.go

@@ -132,6 +132,10 @@ func (n *ndkSingleton) GenerateBuildActions(ctx android.SingletonContext) {
 		}
 	})
 
+	// Include only a single copy of each license file. The Bionic NOTICE is
+	// long and is referenced by multiple Bionic modules.
+	licensePaths = android.FirstUniquePaths(licensePaths)
+
 	combinedLicense := getNdkInstallBase(ctx).Join(ctx, "NOTICE")
 	ctx.Build(pctx, android.BuildParams{
 		Rule:        android.Cat,