Scan APEX packages

* Add a flag to toggle the feature. This makes it easy to compare the
  performance impact with and without the feature. The feature can
  be disabled if any regression is found.
* Add InstallPackageHelper#scanApexPackages to scan APEX packages
  during boot. APEX now goes through the same scan flow as APK.
  Note APEX is not registered with the system and its PackageInfo is
  still cached in ApexPackageInfo.
* For rebootless APEX, #scanSystemPackageLI is applied to the newly
  installed APEX. This is consistent with how we scan APEX during
  boot.

Duration of scanApexPackagesTraced(), measured on Pixel4a:
feature enabled: 110ms
feature disabled: 50ms

This is expected because we now have more packages to scan. We used to
scan APK only. Now both APK and APEX are scanned during boot.

Bug: 225756739
Test: atest com.android.server.pm.ApexManagerTest \
            GtsStagedInstallHostTestCases \
	    CtsStagedInstallHostTestCases \
	    StagedInstallInternalTest
Change-Id: If762100a08c9c59ae55a058f2ca42a06fcc6b80a