update_prebuilts.py: copy library dependencies into updated build tools

The libconscrypt_openjdk_jni dependency of signapk and the
libwinpthread-1 dependency for lld were manually copied into the
build tools, and each time update_prebuilts.py is run they are deleted,
requiring manual restoration.  Preserve them by moving them into the
downloaded artifacts directory before overwriting the tools directory
with the downloaded artifacts.

Bug: 180055535
Test: ./update_prebuilts.py -b 6752562
Change-Id: Icd964ab6a7c45ca3e0d09c70787d024dbe386015
1 file changed