Add InputMethodService#exposeContent()

This is a follow up CL to my previous CLs [1][2] that introduced
InputConnection#commitContent(InputContentInfo, Bundle) API to enable
IMEs to send a content to the target application.

With this CL, IME developers are able to temporarily expose
InputContentInfo object to the target package without permanently
granting URI permission.  Although calling IMS#exposeContent() is
allowed only for the IME that is currently selected, the client is able
to request a temporary read-only access even after the current IME is
switched to any other IME as long as the client keeps InputContentInfo
object.

Here is a sample code snippet about how to use this mechanism.

  [IME]
  InputContentInfo contentInfo = new InputContentInfo(
          contentUri,
          new ClipDescription(description, new String[]{mimeType}),
          linkUrl);
  exposeContent(contentInfo, getCurrentInputEditorInfo());
  getCurrentInputConnection().commitContent(inputContentInfo, null);

  [App]
  try {
      contentInfo.requestPermission();
      // Load inputContentInfo.getContentUri() here.
  } finally {
      contentInfo.releasePermission();
  }

 [1]: Iaadf934a997ffcd6000a516cc3c1873db56e60ad
      152944f4
 [2]: Ica1ba3154795c1bf44e140dfe639b299f83cd8af
      adebb525

Bug: 29450031
Change-Id: I2772889ca01f2ecb2cdeed4e04a9319bdf7bc5a6