Add stagedApexInfos to ApexStagedEvent

Previously, the event struct had the list of staged APEX names. This
leads to calling getStagedApexInfo(name), which in turn calls
getStagedApexInfos(session).

Note that StagingManager gets the list of APEX names by calling
getStagedApexInfos(session). Hence, the same method gets called
repeatedly.

getStagedApexInfos(session) is quite heavy because it involes mounting
APEXes in the session and running an external program.

Passing stagedApexInfos in ApexStagedEvent and providing
getStagedApexInfos() make apexd & observers happy. (no need to run the
same operation over and over.)

Bug: 370712193
Test: StagingManagerTest
Test: StagedInstallInternalTest
Change-Id: I8d6a0baf6caff453f22d87a694a9eae91361a490