Add script to import manifest merger artifacts

Add a copy of prebuilts/tools/import-maven-artifacts.sh customized to
import into prebuilts/manifest-merger and to only copy artifacts in
com.android.* packages.

Test: JAVA_HOME=$ANDROID_BUILD_TOP/prebuilts/jdk/jdk8/linux-x86/ \
        ./import-maven-artifacts.sh com.android.tools.build:manifest-merger:26.4.0-beta05
Change-Id: I035664f433fac7d41d71a905e470c98381549cea