[pm] move extractNativeLibraries out of lock

This operation can take long and block other accesses of mLock such
as getPackageName. Move it out of the synchronized block, and use
mStageDirInUse to prevent the stage dir from being modified.

BUG: 393698598
FIXES: 393698598
Test: presubmit
FLAG: EXEMPT refactor

Change-Id: I9eb5442c5c4e3e86826635d4eb0be2418ee736c5