Use a flag to grant a temporary URI permission.

It turns out that we can let the system to call
InputMethodService#exposeContent(InputContentInfo, EditorInfo), which
added in my previous CL [1], during the IME is calling
InputConnection#commitContent() as follows.

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

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

This gives us flexibility to let InputConnection#commitContent() do all
the magic for IME developers like other APIs such as
Context#startActivity(), rather than asking them to call one more API to
grant a temporary URI permission like a scenario where
Context#grantUriPermission() is used.

 [1]: I2772889ca01f2ecb2cdeed4e04a9319bdf7bc5a6
      25e0813e

Bug: 29450031
Change-Id: I99536cd58c9984af30b0bafb4a1dd25a26634a2d