Fixing NPE when there is no DownloadProvider

If there is no content provider registered for authority 'downloads', or
it is disabled, InstallStart would crash. Fixing it by checking for
null.

Test: Manual.
1. Disable DownloadProvider:
adb shell pm disable com.android.providers.downloads
2. Use ES File explorer (or any other 3p app) to launch an apk install.
No crash - install goes through normally.

Fixes: 73078986
Change-Id: I4876174a7a87530aacaf9f35cca3071fa353b339