Make getUid a method on UserHandle itself instead of static

This commit addresses the API review feedback that getUid()
will be better to make it be a method on UserHandle itself
rather than a static method.

Update as it is and update the corresponding usages.

Fix: 184735865
Test: make update-api
Test: atest FrameworksNetTests
Test: atest CtsNetTestCasesLatestSdk
Change-Id: I33844309224d84764704255d251fadc8940202ca