Add MissingUsesLibraries to Soong vars for scripts that use --skip-make.

In platform builds missing libraries are defined with make variable
INTERNAL_PLATFORM_MISSING_USES_LIBRARIES via soong_config.mk. When Soong
is invoked with "--skip-make" parameter,
INTERNAL_PLATFORM_MISSING_USES_LIBRARIES is not picked up. As a result
the build fails because of missing dependencies.

This CL uses get_build_var to get the value of
INTERNAL_PLATFORM_MISSING_USES_LIBRARIES, then converts it to JSON list.

In the future would be better to migrate both scripts to use --skip-kati
introduced in https://r.android.com/1512613.

Bug: 175286760
Bug: 132357300
Test: forrest build for target "ndk" branch "aosp-master".
Test: Patch build-aml-prebuilts.sh and build-ndk-prebuilts.sh to dump
  their soong.variables config, observe that MissingUsesLibraries is
  coherent with INTERNAL_PLATFORM_MISSING_USES_LIBRARIES. Start running
  each script and observe that they don't fail early due to missing
  dependencies.
Change-Id: I5dbc1d3ea7a64de1e8be6332003acc940c2e6a76
diff --git a/scripts/build-ndk-prebuilts.sh b/scripts/build-ndk-prebuilts.sh
index b6ed659..1a33219 100755
--- a/scripts/build-ndk-prebuilts.sh
+++ b/scripts/build-ndk-prebuilts.sh
@@ -30,6 +30,11 @@
 PLATFORM_VERSION_ALL_CODENAMES=${PLATFORM_VERSION_ALL_CODENAMES/,/'","'}
 PLATFORM_VERSION_ALL_CODENAMES="[\"${PLATFORM_VERSION_ALL_CODENAMES}\"]"
 
+# Get the list of missing <uses-library> modules and convert it to a JSON array
+# (quote module names, add comma separator and wrap in brackets).
+MISSING_USES_LIBRARIES="$(get_build_var INTERNAL_PLATFORM_MISSING_USES_LIBRARIES)"
+MISSING_USES_LIBRARIES="[$(echo $MISSING_USES_LIBRARIES | sed -e 's/\([^ ]\+\)/\"\1\"/g' -e 's/[ ]\+/, /g')]"
+
 SOONG_OUT=${OUT_DIR}/soong
 SOONG_NDK_OUT=${OUT_DIR}/soong/ndk
 rm -rf ${SOONG_OUT}
@@ -49,7 +54,9 @@
     "Safestack": false,
 
     "Ndk_abis": true,
-    "Exclude_draft_ndk_apis": true
+    "Exclude_draft_ndk_apis": true,
+
+    "MissingUsesLibraries": ${MISSING_USES_LIBRARIES}
 }
 EOF
 m --skip-make ${SOONG_OUT}/ndk.timestamp