Only have featureFlag attr in xml when v>baklava

This makes it so that if there are xml files that use the featureFlag
attribute they are split into a pre-B file where all flags are assumed
false and a B version that is the file as is.

Test: Automation
Bug: 377974898
Flag: android.content.res.layout_readwrite_flags
Change-Id: Iab1a69a6d0b3e7efd7033887c351430fb2aabd19