Add tests for InstallSourceInfo.

Check we get the expected installer & initiator for intent- and
session-initiated installs.  (Originator is always null, because we
can't read it without INSTALL_PACKAGES permission.)

Also make class name match file name for
ExternalSourcesTestAppOpAllowed so it can be run via atest.

Bug: 152375595
Test: atest InstallSourceInfoTest
Test: atest CtsPackageInstallTestCases
Change-Id: I6a96f65cf932531d2f88b49f7ff6dc447b1cf9ae