Split getInstance and createInstance in AppSearchImpl

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

Split createInstace for initialization, and getInstance for other calls.
Removed AppSearchException from getInstace's signature.

Test: presubmit
Bug: 162450968
Change-Id: I925506ad0f6540b060c77ed2a535737ee71f781c