Disable armeabi builds for Android and re-enable CI builds. (#4970)

armeabi support was removed from the Android NDK so we should no
longer build it.  Since this fixes the Android build failures this
commit also re-enables Travis Android builds.

While re-enabling Android builds, some recent changes broke C++98
support so this fixes those issues as well which include:
- Conditionally compiling use of move constructors, operators and
  std::move.
- Changing sample to use flatbuffers::unique_ptr rather than
  std::unique_ptr.

Finally, added the special "default_ptr_type" value for the
"cpp_ptr_type" attribute.  This expands to the value passed to
the "--cpp-ptr-type" argument of flatc.

tests/test_builder.cpp

@@ -1,3 +1,5 @@
+#include "flatbuffers/stl_emulation.h"
+
 #include "monster_test_generated.h"
 #include "test_builder.h"
 
@@ -12,13 +14,20 @@ struct OwnedAllocator : public flatbuffers::DefaultAllocator {};
 
 class TestHeapBuilder : public flatbuffers::FlatBufferBuilder {
 private:
+  // clang-format off
+  #if !defined(FLATBUFFERS_CPP98_STL)
   TestHeapBuilder(const TestHeapBuilder &);
   TestHeapBuilder &operator=(const TestHeapBuilder &);
+  #endif  // !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
 
 public:
   TestHeapBuilder()
     : flatbuffers::FlatBufferBuilder(2048, new OwnedAllocator(), true) {}
 
+  // clang-format off
+  #if !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
   TestHeapBuilder(TestHeapBuilder &&other)
     : FlatBufferBuilder(std::move(other)) { }
 
@@ -26,6 +35,9 @@ public:
     FlatBufferBuilder::operator=(std::move(other));
     return *this;
   }
+  // clang-format off
+  #endif  // !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
 };
 
 // This class simulates flatbuffers::grpc::detail::SliceAllocatorMember
@@ -49,12 +61,18 @@ public:
     Swap(other);
   }
 
+  // clang-format off
+  #if !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
   GrpcLikeMessageBuilder &operator=(GrpcLikeMessageBuilder &&other) {
     // Construct temporary and swap idiom
     GrpcLikeMessageBuilder temp(std::move(other));
     Swap(temp);
     return *this;
   }
+  // clang-format off
+  #endif  // !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
 
   void Swap(GrpcLikeMessageBuilder &other) {
     // No need to swap member_allocator_ because it's stateless.