Feature add provide Java Graph class getter for all operations (#12243)

* . c_api code and tests to support exposing the cardinality of operations within a graph, and populating a parameter of TF_Operations** with all operations within a graph.
. JNI code to take advantage of this new functionality.
. Java code and tests to expose the JNI layer in the Graph class

* . Changes in response to @asimshanka feedback.

* . Protect against the case in which the iteration is actually stuck returning the same operator from the graph.

* Style tweaks

- Tweak comment to make for a shorter summary fragment (https://google.github.io/styleguide/javaguide.html#s7.2-summary-fragment)
- Apply the formatter: https://github.com/google/google-java-format

* graph_jni.h: Formatting fix

* Graph.java: Formatting change

* . Better treatment of the pos variable; line re-organization per suggestions from @asimshankar