Scan package to derive ABIs before optimizing.

This requires a minor refactor to extract the ABI detection logic
out of scanPackageDirtyLI.

Note that there's a minor regression here : we ignore the
cpuAbiOverride from the package settings when calculating the
CPU ABI. This is OK (and possibly better behaviour) because this
is only a debug only option (for adb install) AND because the instructions
require users to specify the abi override on every adb install
invocation. Furthermore, the behaviour when an ADB installed app
(with an override) is auto-updated is more consistent.

bug: 21144503
Change-Id: I74e7c493468ee0088eb615c9a7fe30b4d7cf27de