AAPT2: Ignore namespaced elements in AndroidManifest.xml

Some third party stores/tools expect manifest elements
under their namespace, and AAPT2 shouldn't fail if these
are present.

Bug: 37943705
Test: make aapt2_tests
Change-Id: I87b7500c7da5e8e79fc2a78b30e8e4334124af3d