Do not make strong reference to Resources.

This CL removes the strong reference added for mapping display ids
and Resources to Displays. Instead, the key pair is now the display
id and ResourcesKey, and the mapping is pruned when key is
invalidated.

Change-Id: I60d767b52de7bbf769f6761f5a3301dd7aff6ddf
Fixes: 36625868
Test: make -j32 cts; cts-tradefed; run cts --module CtsAppTestCases --test android.app.cts.DisplayTest#testRotation