Set MIME type when building BROWSE intents.

If we created a BROWSE PendingIntent early during boot (while the
device was still locked), we would resolve the MIME type as "null"
meaning the intent would fail to launch.

Since we always know what the MIME type will be, set it explicitly.

Test: boot device while USB device attached
Bug: 30742481
Change-Id: I23c523cbe2660ca63a3b7df1f41300ab803b314c