Add AppWidgetManager#getInstalledProvidersForPackage()

This allows more fine-grained control than getting all installed
providers for a user, when you might only want to check for a particular
package. For instance, Launcher can use this API to surface widgets per
app without having to ask for all the widgets.

Test: Unit test on AppWidgetServiceImplTest
$ runtest --path=services/tests/servicestests/src/com/android/server/appwidget/AppWidgetServiceImplTest.java

Bug: 34940468
Change-Id: I182bf1c012d31182024422fc4a63f57f151c3ee5