Copy StringPool to framework

Libcore class StringPool has single usage outside libcore, in framework.
As a part of api cleanup in libcore StringPool is removed from
SystemApi, thus, it is copied to com.android.internal.util package.

Bug: 154796679
Test: m droid
Test: atest FrameworksUtilTests
Change-Id: Iaa332eb8ade68f3451639bdce62d5cfea30c2f97