Simplify flags used to wait for staged session ready

Staged install by default should wait for staged session to become
ready. The default waiting time can now be altered using a single flag
--staged-ready-timeout. Passing non-positive integer as argument for the
flag turns off the waiting logic, which is equivalent to --no-wait flag
from before.

Tests have been updated accordingly to match the change.

Bug: 162958790
Test: atest StagedInstallInternalTest
Change-Id: I998cf6882a6b38547c2283f6cadc41082c25acb5