Cleanup Apex E2E-test base class

It didn't really lend itself well to extension, so refactor the code to
have util methods like install, reboot, uninstall that subclasses can
use to write more involved test cases.

While here, make some other improvements:
- remove double-triggering of additionalCheck()
- use Truth consistently instead of a weird mix
- other readability improvements, like shortening lines, tweaking msgs

Bug: 149815613
Test: atest sdkextensions_e2e_tests
Change-Id: I1900358743eb34282682608e04b6b9fe0f36f11c
1 file changed