Fix AssetManager2 isUpToDate check

This logic was lost in the AssetManager1 -> 2 migration.

The old AM1 checked the last modification time of the file
and compared it to a previously stored value. This re-adds the
logic to ApkAssets and fixes the checks in the JNI/Java layer.

Unfortunately I couldn't find a failing/practical case where
this check mattered. It only came up when diagnosing an issue
which ended up being unrelated.

Test: manually ran with other overlay changes

Change-Id: I758e4af1d32a9c03b2204a8a3a26e82b7e83feda