Fix minSdkVersion scanning to not throw warnings

For the UTF8/UTF16 switch code, we needed to know what was the
minSdkVersion specified as early as possible. Unfortunately, this threw
warnings when the SDK was compiling since we always set this field in
the Bundle.

This splits out the field used by the initial AndroidManifest.xml scan
to a separate one that we won't attempt to re-insert into the
AndroidManifest.xml  This also switches the logic to better reflect the
preference of UTF-8 over UTF-16; previously UTF-16 was the default.

Change-Id: Ia81f6b21047043ebb711eb24c2c3718534979ef6