Clarify InstallSourceInfo javadoc.

Added comments to better explain:
- How initiating differs from installing;
- When null might be returned from either (expanding on existing
  comments on the PM#getInstallSourceInfo method).

Bug: 150330937
Test: Still builds.
Change-Id: I03c5d92f643e97019b00778ca1d8e0c92e25dac8