Document system activity intents that may not be implemented

All of these activity-start intents might be unimplemented on some
Android products.  Document this to make sure that developers are
aware of the need to safeguard against this.

Bug: 68300743
Bug: 62201251
Bug: 69587018
Fixes: 77282739
Test: atest CtsContentTestCases:.AvailableIntentsTest
Change-Id: Ia2346d03ccb7f2bdad2b84ba9efff72413fdc3c2