Support an ABI flag for instrumentation.

Allows us to choose what ABI a process uses when
launching it with "adb shell am instrument", for eg.

adb shell am instrument --abi arm64-v8a component/runner

Note that we only perform very basic validation of the
ABI. In general, there is no guarantee that the app will
launch with the instruction set we choose, for eg. if it
has native libraries that are for a different ABI.

bug: 14453227

Change-Id: Ifb7e89b53675080dc87941091ee5ac360f218d7f