Don't attempt to write to / for aar prebuilts

This path needs to be set if we're going to build aar prebuilts with
aapt2.

Bug: 33381544
Test: Attempt to build an aar prebuilt with LOCAL_USE_AAPT2 := true, it
      doesn't fail writing to /

Change-Id: I28c85736986221607a1d7a295ff1522b993a2049