Avoid ClassCastException in ArrayMap.

Only happens if you're put()ing and clear()ing the map from
different threads, and Dianne told you not to do that.

In addition to avoiding the cache poisoning that results
from concurrent access, ArrayMap now attempts to throw
ConcurrentModificationException if clear() or
ensureCapacity() or put() notices you've modified the map
elsewhere.

Bug: 32994281
Test: runtest -x frameworks/base/core/tests/coretests/src/android/util/ArrayMapTest.java
      runtest -x cts/tests/tests/util/src/android/util/cts/ArrayMapTest.java
Change-Id: Ia75970aa9e2b2b65692179f51243584b9773797f