[pm] reduce lock contention on createSession

mInstallerUid doesn't need to be locked and it has been changed to volatile since ag/14040815. Removing the synchronized block so that the creation of one session is not blocked by the validation of another.

Change-Id: Ife08af651656a81a59be0c1ffb2a30a84e390f8c
BUG: 319178414
FIXES: 319178414
Test: n/a
FLAG: EXEMPT bug fix