[pm] unlock mInstallLock before dexopt

Dexopt using ArtService doesn't need to be locked by the mInstallLock,
as it no longer depends on installd and can handle I/O failures
gracefully. Unlocking it can prevent crashes due to lock contention on
mInstallLock during long dexopt times.

BUG: 306917971
Test: first make sure dexopt is enabled by setting "dalvik.vm.disable-art-service-dexopt" to false.
Test: install the gmscore apk and while it's installing,
creating/switching users by using Guest users. Observe in the log that
the mInstallLock can be locked and unlocked while dexopt is still going
on.

Change-Id: Ibe283cb0f9057e6af336a6733bf20efa7ff36ef3