Don't export HeapDumpProvider.

Stop exporting HeapDumpProvider so apps can only access generated dumps
when the user explicitly shares them.

Bug: 184046948
Test: capture system heap dump in developer options and confirm test app
get SecurityException if it tries to access the dump directly, but gets
access when the dump is shared through the notification flow

Change-Id: Ibdca7cde4f563baa39163869289da5b79fc3a6db