install: Return bool for a few check functions.

The results from these functions have boolean semantics. They're
returning `int` prior to this CL, with some of them mixing 0 and
InstallResult.  Note that SetUpNonAbUpdateCommands() was returning
INSTALL_CORRUPT / INSTALL_ERROR / 0 prior to this change, but all the
callers handle INSTALL_CORRUPT and INSTALL_ERROR the same way.

This CL changes them to return bool instead.

Test: `mmma -j bootable/recovery`
Test: TreeHugger
Test: Sideload on taimen.
Change-Id: Ic1b5dbf79aaca68b53ab8ea2c8ba3d19f988c571