PackageManager: Support secondary ABI for native code at installation time.

The goal of this change is to allow the Package Manager to look for native
shared libraries corresponding to the CPU ABI reported by ro.product.cpu.abi2
if none was found for the CPU ABI given by ro.product.cpu.abi

This is used to support both ARMv5 and ARMv7 native code on modern ARMv7-based
devices. Typically, such a device would report a primary ABI of 'armeabi-v7a'
and a secondary one of 'armeabi', to indicate that it can run the binaries
generated for both.