Register the DexLoadReporter before creating the main class loader

We used to register the DexLoadReporter after the main class loader
was created. That was done to avoid superfluous reports of the app's
primary apks (since we already know about them).

However that also means that if the app would start as an isolated
process we would miss this signal in the DexManager.

Make sure we get signals for isolated processes by registering the
reporter before we create the main class loader.

Test: manual, using chrome and maps go
adb shell cmd package compile -m speed-profile com.android.chrome
adb shell am start -n \
com.google.android.apps.mapslite/com.google.maps.lite.twa.MapsLiteTwaLauncherActivity
adb shell dumpsys package dexopt
(confirm that chrome is used by an isolated process)
adb shell cmd package compile -m speed-profile com.android.chrome
(confirm that chrome is speed compiled - or verify, based on the device
setting)

Bug: 163018062
Change-Id: I5d683ae85b3973cc2011dd33b4cb14837b18005f