Fix backwards compat problem with AAPT public attrs

AAPT has traditionally assigned resource IDs to public attributes,
and then followed those public definitions with private attributes.

--- PUBLIC ---
| 0x01010234 | attr/color
| 0x01010235 | attr/background

--- PRIVATE ---
| 0x01010236 | attr/secret
| 0x01010237 | attr/shhh

Each release, when attributes are added, they take the place of the private
attributes and the private attributes are shifted down again.

--- PUBLIC ---
| 0x01010234 | attr/color
| 0x01010235 | attr/background
| 0x01010236 | attr/shinyNewAttr
| 0x01010237 | attr/highlyValuedFeature

--- PRIVATE ---
| 0x01010238 | attr/secret
| 0x01010239 | attr/shhh

Platform code may look for private attributes set in a theme. If an app
compiled against a newer version of the platform uses a new public
attribute that happens to have the same ID as the private attribute
the older platform is expecting, then the behavior is undefined.

We get around this by detecting any newly defined attributes (in L),
copy the resource into a -v21 qualified resource, and delete the
attribute from the original resource. This ensures that older platforms
don't see the new attribute, but when running on L+ platforms, the
attribute will be respected.

We still need to address this problem in the platform moving forward,
as this will only help us in the transition from pre L to L.

Bug:17520380
Change-Id: Ia2a985798b50006c21c7c3431d30d9598f27cd91