Remove/document @hidden APIs in Canvas

Bug: 155422223
Test: make

For Canvas to be in a mainline module, it cannot be have @hidden APIs
accessed from outside the module. Remove @hide where simple:

- document why Canvas(long) is needed to be public + @hide
  - remove its UnsupportedAppUsage, which was a false positive
- remove EdgeType.nativeInt (which was @hide) - no longer used
- make VertexMode.nativeInt package protected

Change-Id: I508758889c586f336c2963e1b61d01dc3dc58be7