Apply a maximum char count to the load label api

The system is overwhelmed by an enormous label string returned by
the load label api. This cl truncates the label string if it exceeds
the maximum safe length.

Bug: 67013844
Bug: 197298717
Test: atest PackageManagerTest
Change-Id: Ia4d768cc93a47cfb8b6f7c4b6dc73abd801809bd
Merged-in: Ia4d768cc93a47cfb8b6f7c4b6dc73abd801809bd