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This CL completes the basic implementation of the flags UI. During
startup, the embedded WebView implementation will check if the user has
enabled developer mode, and if so, fetch the flag overrides from the
service.

This uses a ContentProvider instead of an aidl method on the service
interface for the sake of a simple synchronous IPC. Although aidl itself
supports synchronous IPC, the Android framework only supports binding to
the service asynchronously. We require fully synchronous IPC so we can
block startup while we fetch the flags.

Now that we've settled on using a ContentProvider to plumb information
from the developer UI to embedded WebViews, this changes "developer
mode" to be defined by the ContentProvider's state rather than the
Service's state, which has the side benefit of simplifying some of the
Activity/Service code.

We rely on PackageManager APIs to check if developer mode is enabled.
The check itself should have very little impact to startup time, since
PackageManager caches its state in RAM. I benchmarked this check (when
developer mode is disabled) at 0ms on my Google Pixel 2 device. For
simplicity, we do not care about performance when developer mode is
enabled, as this is not the usual user experience.

Bug: 981143
Test: Manual - toggle debug border flag, start WebView shell, see borders
Test: Benchmark isDeveloperModeEnabled() with System.currentTimeMillis()
Test: run_android_webview_junit_tests -f *ServiceNamesTest*
Change-Id: I7cc67d1bdf8f0f2ce0fce714fb359160899354a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1977828
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726993}