Make the StartInstall abort-able

The StartInstall may take more than one mintue. This CL adds an
aborting_ flag to notify the allocating procedure to exit once
the flag is set. It also adds a "cancel" command to gsi_tool for
testing purpose.

Test: open two adb consoles: one starts the installation and
    another uses the cancel command to abort.
Bug: 125599267
Change-Id: I8f8fc460df526abda81ed6e0ac8f7176a28e9df3
3 files changed