Fix for client tracking on bind to service.

We were not correctly updating the bound client UIDs when binding to a
service with an already-running process. This could result in a
background activity start by the app hosting the service being
incorrectly blocked.

(Also includes a drive-by fix of a potential NPE.)

Bug: 131468397
Test: Manually verified YouTube VR problem is fixed.
Test: atest BackgroundActivityLaunchTest
Test: atest RootWindowContainerTests
Test: atest WmTests:ActivityStarterTests
Test: atest CtsWindowManagerDeviceTestCases:ActivityStarterTests
Test: atest CtsAppTestCases:.ServiceTest
Change-Id: I153cf3678f10439075af39a8bd88ff8d492e1ffe