Adding an api for apps to check whether they can install apps

Some apps may want to check whether they are trusted to install apps on
the device, so they can prompt the user to go to settings and mark them
as trusted before they do an intensive operation like downloading an
apk.

Test: cts-tradefed run cts -m CtsExternalSourcesTestCases

Bug: 31002700
Change-Id: Icd9d04daa157e6733decba245ec251ce4acd4122