AAPT: Continuation of public/private attribute fix

XML files like layouts are now scanned and checked
for v21 attributes. If those kinds of attributes
are found, then we remove them in the original
version and synthesize a new xml file under the
v21 configuration.

Bug:17520380
Change-Id: Icf984cb96134180a2e35349c1dbf2cef9a8f0bda