Fix onPackageChanged handling of disabled packages

TransportManager#onPackageChanged was not correctly handling the
circumstance where broadcast ACTION_PACKAGE_CHANGED was received
with EXTRA_CHANGED_COMPONENT_NAME_LIST containing only the
package name, instead of >=1 component names.  When that happens
it indicates that a package-wide change has occurred, such as
the package being enabled or disabled.

If the package in question contains backup transports, they
should be unregistered if the package is disabled, and re-
registered if the package is enabled.  But the current
TransportManager#onPackageChanged doesn't know enough to do so.

We can determine the enabled state of the package by calling
PackageManager#getApplicationEnabledSetting. This commit modifies
onPackageChanged to do that, and then (un/re)-register the
packaged transports as appropriate.

To test I extend the Roboelectric ShadowApplicationPackageManager
so that the {get,set}ApplicationEnabledState() methods are
exposed for use in the test setup.

Bug: 162725876
Fixes: 162725876
Test: 1. atest -v TransportManagerTest
      2. manual test on device:
           1 device:/ # bmgr list transports
           2 device:/ # pm disable com.google.android.gms
           3 device:/ # bmgr list transports
           4 device:/ # pm enable com.google.android.gms
           5 device:/ # bmgr list transports
         observe logcat showing BackupTransportManager MORE_DEBUG:
	   step 2: Package c.g.a.gms was disabled.
	   step 4: Package c.g.a.gms was enabled.
	           Transport c.g.a.gms/.b.c.D2dTransportService registered
Change-Id: I7bd397f0f1eadbbca6f371c3c9710df5386aca13