Add an option to specify the path to aapt2.

Thin manifests (e.g. master-art) may not have the aapt2 sources and
instead relies on the prebuilt in prebuilts/sdk/tools/linux/bin. In
that case there's no aapt2 in the search path, so it needs to be
specified.

Test: Heavy presubmit build on mainline_modules_bundles on git_master
Test: vendor/google/modules/ArtGoogle/build-art-module.sh
  with http://ag/16584845 on git_master-art, where only the prebuilt
  aapt2 is present
Bug: 212462945
Change-Id: I41a22e8146f5a88534c2721345b4d9d64f76698c