StatsService allows uids to impersonate themselves

Previously, most StatsService calls only allow the caller to specify the
uid if it is Userdebug/EngBuild. This applied even if the caller was
just specifying its own UID; this cl allows such a case. It also allows
ROOT to impersonate SHELL.

Test: Manual testing on userdebug and userbuild devices.
Test: make statsd_test && adb sync data && adb shell data/nativetest64/statsd_test/statsd_test

Change-Id: I2685d13a8dc24d40b5dab8be9842f53be6888ba3