AAPT2: Flatten AndroidManifest.xml stringpool in UTF16

A few OEM devices have introduced a memory corruption bug
that is only triggered when an application's AndroidManifest.xml
uses UTF8 string pools. Workaround this by only encoding
AndroidManifest.xml with UTF16 strings.

Bug: 64434571
Test: manual
Change-Id: I2ee50d1b2b5942d971ef2a544c878de63f67c652