Swap default value for Player PlayOnlyInForeground property

[ewpatton]

General items:

• I have updated the relevant documentation files under docs/
• My code follows the:
  • Google Java style guide (for .java files)
  • Google JavaScript style guide (for .js files)
• 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 appinventor/components/src/.../common/YaVersion.java
• I have updated the corresponding upgrader in appinventor/appengine/src/.../client/youngandroid/YoungAndroidFormUpgrader.java (components only)
• I have updated the corresponding entries in appinventor/blocklyeditor/src/versioning.js

What does this PR accomplish?

As discussed at the dev planning meeting, the default value of the PlayOnlyInForeground property made our existing static analysis for whether to apply permissions not work w.r.t. iOS. This PR changes the default value so that the lack of this property in the code guarantees that an app built for iOS can rely on its presence to set the audio background mode property. The presence of this flag is scrutinized and the current system causes people using the Player in published apps to have to justify the audio background mode's presence.