Wait for header contents loaded with backoff timeout

The first time a test opens a target activity it would take
longer to load content completely, and may cause a test to
fail with timeout.
Now it will attempt loading three times with backoff timeout
each retry.

Test: refactoring CL. existing TV ABVT will pass
Bug: 33472111
Change-Id: I9faf5459552b366950a9bd9c5909dc442cfe9b71