Finish migrating KeyAgreementActivity to ViewModel

The code has been refactored substantially (though almost all functionality should have stayed the same), so a faster way to review the code might be to check out the nearby package in Android studio and otherwise look at the diffs of the individual commits.

Test instructions:

• Add two contacts with the nearby option
• try all sorts of variations and complications when doing it:
  • revoke permissions in system settings for briar
  • rotate screen in odd places through the process (especially before pressing the Next button)
  • disable wifi and/or bluetooth of the phone and/or in briar itself (before starting to add contacts)

Closes #1872 (closed)

.idea/inspectionProfiles/Project_Default.xml

+<component name="InspectionProjectProfileManager">
+  <profile version="1.0">
+    <option name="myName" value="Project Default" />
+    <inspection_tool class="WeakerAccess" enabled="true" level="WARNING" enabled_by_default="true">
+      <option name="SUGGEST_PACKAGE_LOCAL_FOR_MEMBERS" value="true" />
+      <option name="SUGGEST_PACKAGE_LOCAL_FOR_TOP_CLASSES" value="true" />
+      <option name="SUGGEST_PRIVATE_FOR_INNERS" value="true" />
+    </inspection_tool>
+  </profile>
+</component>
\ No newline at end of file