Add proto property struct

The ProtoProperties struct was not being added to the property struct
list, preventing proto.type and proto.static properties from being set
in Android.bp files.

Bug: 32286026
Test: Add proto.type to system/extras/perfprofd/Android.bp, run soong
Change-Id: I9a5b63ddd3e7a53881ddbfd381c42aa1b4c4f454