Move more Omaha tests to become Robolectric tests

This change moves the ExponentialBackoffSchedulerTest,
RequestGeneratorTest, and OmahaBaseTest to be based on Robolectric.

Multiple files depended on MockExponentialBackoffScheduler,
MockRequestGenerator, and AttributeFinder, so instead of doing
gymnastics with temporary putting these in a shared GN test support
target, these test fixes are submitted together.

Using Robolectric requires stopping all usage of AdvancedMockContext,
though InMemorySharedPreferencesContext is OK. However, Robolectric
provides similar functionality as AdvancedMockContext through
RuntimeEnvironment.getApplication().

The OmahaBaseTest directly invokes checking for updates, which should
not be invoked on the main thread. However, in this very targeted test
it is OK to do so, but the thread asserts still fail. This change
therefore changes to do the assert through TestUtil, and creates
an AutoCloseable to ensure as little test code as possible is affected
by disabling thread asserts.

R=waffles@chromium.org

Bug: 1332190, 1332191, 1332192
Change-Id: I6687ec2bff7180ba3a4566ebe9b8a78f34f11ada
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3705785
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1014921}