Fix hint text not showing in MockTextBoxBase

[ewpatton]

Change-Id: Ie3b373eea228d81e5de55d917a6606f649e63ed0

General items:

• [ ] I have updated the relevant documentation files under docs/
• [x] My code follows the:
  • [x] Google Java style guide (for .java files)
  • [ ] Google JavaScript style guide (for .js files)
• [x] ant tests passes on my machine

If your code changes how something works on the device (i.e., it affects the companion):

• [ ] I branched from ucr
• [ ] My pull request has ucr as the base

Further, if you've changed the blocks language or another user-facing designer/blocks API (added a SimpleProperty, etc.):

• [ ] I have updated the corresponding version number in YaVersion.java
• [ ] I have updated the corresponding upgrader in YoungAndroidFormUpgrader.java (components only)
• [ ] I have updated the corresponding entries in versioning.js

For all other changes:

• [x] I branched from master
• [x] My pull request has master as the base

What does this PR accomplish?

This PR causes changes to the Hint and HintColor properties to be accurately reflected in MockTextBoxBase and its descendants.

Fixes #3236