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