customtabs: Add convenience methods to get the package name and warmup().

To follow best practices, Custom Tabs clients have to connect to the
warmup service, and warm up the implementation. Also, finding the right
package to direct the intents to has to be implemented by clients.

This adds two convenience methods:
- Find the right package name.
- Connect to the service, and warm up the implementation in a "fire and
  forget" fashion.

Change-Id: If55ff76327bbf0a36b9b133becaab809d381d628
2 files changed