Use the @Nullable annotations for choosePrivateKeyAlias.

Both the code and docstring support this, but the parameters weren't
annotated.

Test: it builds locally

Change-Id: I16beddcd74a86047ce9aaf37007d96f3e8e0d4e0
Merged-In: I16beddcd74a86047ce9aaf37007d96f3e8e0d4e0
Fix: 78868934
(cherry picked from commit b7c5eddc53c3872b661222ae30270d95cfe63b4e)
1 file changed