Enable instrumentation of system server on debuggable devices.

This makes two changes to enable the instrumentation of system server
and other system processes:

- A new option '--no-restart' has been added to 'am instrument', that
causes the test apk to be loaded without restarting the target app.

- On debuggable devices, the check that the test apk has the same
signature as the target app is not performed.

With these changes, a test apk with instrumentation configured
with targetPackage="android" can run a test from within the system
server process, on debuggable devices. These options may also allow
other system processes to be tested.

See go/internal-api-testing for more information.

Test: atest FrameworksInProcessTests
Change-Id: I8829bcbe7f3373170bcf1e7fa869d5d31f40576d