Remove raw values from proto APK

Raw attribute values are not included in xml files when building binary
APKs but they are when building proto APKs. Remove raw attribute values
and create a new convert flag that (--keep-raw-values) that allows raw
value to be kept.

Also added flag to link to keep raw attribute values.

Bug: 115915884
Bug: 116700688
Test: aapt2_tests
Change-Id: Id50136b775b3f43d159ca06181282f6a7b8eaf40