AAPT2: Suppress warnings from libandroidfw when built for host

Certain error cases are expected (like not finding a package) when
using libandroidfw on the host side for AAPT2. Suppress those warnings
and let AAPT2 handle error cases.

Change-Id: I3de97128686c03c89c5b99559bf46d4f2f1dfe4d