Split getAppSearchImpl and createAppSearchImpl in AppSearchImpl

User could never call an uninitialized SearchSession, so convert and
propagate the initialize error in getAppSearchImpl to user
for every call is unnecessary.

Test: presubmit
Bug: 162450968
Change-Id: Ie2b3c7c0a56852d7e517c616960c19b7b9e9ad35
3 files changed