Cache more users in UserHandle.of()

We pass around more and more UserHandle these days because of mainline
modules, because even @SystemApi are not allowed to use integer user
IDs.

So let's make sure even in extreme cases we don't keep allocating
new UserHandle's for the same user.

Test: atest $ANDROID_BUILD_TOP/frameworks/base/core/tests/coretests/src/android/os/UserHandleTest.java
Test: atest $ANDROID_BUILD_TOP/cts/tests/app/src/android/app/cts/UserHandleTest.java
Change-Id: Iae34e1d11638ced53246d68d310ad62e058dba93