[pm] refactor InstallArgs

1/n of install flow refactor

Made InstallArgs a final data class and moved all methods to caller classes.

+ Move processInstallRequests to InstallingSession which will become the
entry point of installation requests.
+ Methods that involve mLock or mInstallLock are still kept in
  InstallPackageHalper and RemovePackageHelper.
+ Some errorprone cleanup.

BUG: 238678399
Test: presubmit
Change-Id: I1216cf8254711cc68a50806b7229a63f5df387bd