Add script for building all target arch's needed in AML (Android Mainline)

prebuilts.

This runs Soong in skip-make mode, using normal in-make mode only to query
platform versions.

The same ${OUT_DIR} cannot be used for both skip-make and in-make builds,
because Soong generates a smaller build.ninja file in in-make builds where
many build targets are expected to be provided by the mk files. Thus this
script avoids using ${OUT_DIR} if it's an in-make build, defaulting instead
to out-aml/.

The script is based on build-ndk-prebuilts.sh, but uses a separate Soong
variable Aml_abis to enable the appropriate target architectures for
Mainline modules. Aml_abis is very similar to Ndk_abis, except "armeabi-v7a"
is used instead of "armeabi", which is necessary to match prebuilt
dependencies, e.g. for LLVM.

Test: build/soong/scripts/build-aml-prebuilts.sh libart libdexfile_external
  (verify that libraries for arm, arm64, x86, x86_64 are built)
Test: build/soong/scripts/build-aml-prebuilts.sh \
  out-aml/soong/.intermediates/external/conscrypt/conscrypt-module-sdk/android_common/conscrypt-module-sdk-current.zip
  (verify that the zip file contains libconscrypt_jni.so's for all four arches)
Test: build/soong/scripts/build-aml-prebuilts.sh com.android.art.{release,debug,testing,host}
  (verify that the build completes)
Test: Two identical build/soong/scripts/build-aml-prebuilts.sh runs after each other
  (verify that the 2nd run completes both Soong and ninja steps quickly without any building)
Change-Id: I35712f9f8f0b1cbb77107314c5927c6720e6c3bf