[XAT.Bytecode] Add support for reading NotNull metadata into api.xml.

[jpobst]

First part of #468.

This adds support for reading @NotNull/@NonNull annotations from Java and putting them into api.xml. It does not add logic to consume them.

Adds not-null attribute to api.xml for Fields and Parameters:

<field 
  deprecated="not deprecated" 
  final="true" 
  name="CREATOR" 
  jni-signature="Landroid/os/Parcelable$Creator;" 
  not-null="true" 
  static="true" 
  transient="false" 
  type="android.os.Parcelable.Creator" 
  type-generic-aware="android.os.Parcelable.Creator&lt;android.app.DirectAction&gt;" 
  visibility="public" 
  volatile="false" />

<parameter 
  name="handler" 
  type="android.os.Handler" 
  jni-type="Landroid/os/Handler;" 
  not-null="true" />

Adds return-not-null attribute to api.xml for Methods:

<method 
  abstract="false" 
  deprecated="not deprecated" 
  final="true" 
  name="getContext" 
  jni-signature="()Landroid/content/Context;" 
  bridge="false" 
  native="false" 
  return="android.content.Context" 
  jni-return="Landroid/content/Context;" 
  static="false" 
  synchronized="false" 
  synthetic="false" 
  visibility="public" 
  return-not-null="true" />

Supported Annotations

Android:
android/annotation/NonNull
androidx/annotation/NonNull
androidx/annotation/RecentlyNonNull

Java:
javax/validation/constraints/NotNull
edu/umd/cs/findbugs/annotations/NonNull
javax/annotation/Nonnull
org/jetbrains/annotations/NotNull
lombok/NonNull
android/support/annotation/NonNull
org/eclipse/jdt/annotation/NonNull

List partially taken from https://stackoverflow.com/questions/4963300/which-notnull-java-annotation-should-i-use.