Make `Map.keySet()` handle multiple argments to `@KeyFor`

[mernst]

Currently, the return type of Map.keySet() is Set<@KeyFor("this")>, with this appropriately viewpoint-adapted.
However, the keys should be "this" and all the keys on the receiver's key type.

A test case appears at checker/tests/nullness/KeyForMultiple.java

The current output is

$ch/bin/javac -processor nullness KeyForMultiple.java
KeyForMultiple.java:21: error: [assignment.type.incompatible] incompatible types in assignment.
        Set<@KeyFor({"sharedBooks", "sharedCounts1"}) String> otherChars1 = sharedCounts1.keySet();
                                                                                                ^
  found   : Set<@KeyFor("sharedCounts1") String>
  required: Set<@KeyFor({"sharedBooks", "sharedCounts1"}) String>
KeyForMultiple.java:39: error: [assignment.type.incompatible] incompatible types in assignment.
        Set<@KeyFor({"sharedBooks", "sharedCounts2"}) String> otherChars2 = sharedCounts2.keySet();
                                                                                                ^
  found   : Set<@KeyFor("sharedCounts2") String>
  required: Set<@KeyFor({"sharedBooks", "sharedCounts2"}) String>
2 errors

(This seems to be the same as issue #414, but that issue was closed as fixed, and the problem still exists.)

[Gaoyan1999]

I will try to fix it by overriding addComputedTypeAnnotations in KeyForAnnotatedTypeFactory