[ART tools] Fix construction of a make target string to build public.libraries.txt

As a result of target testing on Pixel 2 it is found that a make target
string to build public.libraries.txt can be incorrect.
For example, when a target is aosp_walleye-userdebug
TARGET_PRODUCT is set to aosp_walleye but ANDROID_PRODUCT_OUT
is set to <absolute path>/out/target/product/walleye. So buildbot-build.sh
creates the make target ‘out/target/product/aosp_walleye/
system/etc/public.libraries.txt’ which is not correct. This results a
ninja failure: unknow target.

The current CL uses ANDROID_PRODUCT_OUT and ANDROID_BUILD_TOP to construct
a make target string.

Test: lunch hikey960-userdebug && art/tools/buildbot-build.sh --target
Test: lunch aosp_walleye-userdebug && art/tools/buildbot-build.sh --target
Test: lunch aosp_taimen-userdebug && art/tools/buildbot-build.sh --target
Test: lunch aosp_x86-eng && art/tools/buildbot-build.sh --target
Test: lunch aosp_arm64-eng && art/tools/buildbot-build.sh --target
Test: lunch aosp_arm-eng && art/tools/buildbot-build.sh --target
Change-Id: I9c3abcd44310598b56800773893420e1c433553f