Add a flag to always do the cert signature check

Right now signature check is unconditionally bypassed on debuggable
builds, which makes things a little bit confusing - instrumentation will
succeed on debuggable build, but fail on the user build.

Bug: 209061624
Test: adb shell am --always-check-signature com.android.tests.supplemental.process/androidx.test.runner.AndroidJUnitRunner
Change-Id: I66d6331156d4503e147141872cbe73a9330ea701