Fix a no-op uninstall being treated as a failure

Treat a stageUninstall() doing nothing as a success, not a
failure. This prevents the system retrying the uninstall
later.

Unit tests run with:

make -j30 FrameworksServicesTests
adb install -r -g \
  "out/target/product/marlin/data/app/FrameworksServicesTests/FrameworksServicesTests.apk"
adb shell am instrument -e package com.android.server.timezone -w \
  com.android.frameworks.servicestests \
  "com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner"

Test: See above
Test: Manual testing, adb dumpsys timezone
Test: PTS: run pts -m PtsTimeZoneTestCases
Bug: 65657176
Change-Id: Ifd205da90e848552711ac3f4207cd22ad2c2747b