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.

samples/android/build.gradle

@@ -66,7 +66,7 @@ android {
       ndkBuild {
         targets "FlatBufferSample"
         arguments "-j" + Runtime.getRuntime().availableProcessors()
-        abiFilters "armeabi", "armeabi-v7a", "arm64-v8a", "x86", "x86_64"
+        abiFilters "armeabi-v7a", "arm64-v8a", "x86", "x86_64"
       }
     }
   }