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The chrome split can take a long time to load, so we get significant
improvements in startup time if this work is offloaded to a background
thread. Ideally, we would give as much time as possible to allow this
preload to finish, which means running as much code as possible which
does not depend on the chrome split during the preload. This required
refactoring much of attachBaseContext into the base module, and moving
the pieces which could not be easily moved to the base module into
onCreate.

The order of startup generally goes:
1. Application.attachBaseContext
2. Each provider is instantiated in order, and onCreate is called
3. Application.onCreate

If a provider lives in a split, it will call createContextForSplit on
the application. This will end up being the first thing needing the
chrome split, which will wait for the preload to complete. This is why
ChromeFileProvider was moved to the base module, so there could be more
time to preload.

Pinpoint runs:
Pixel 2: https://pinpoint-dot-chromeperf.appspot.com/job/16925180d20000
-7% messageloop_start_time
-3% first_contentful_paint_time
-3.8% navigation_commit_time

Android Go: https://pinpoint-dot-chromeperf.appspot.com/job/115e53e7520000
-18.7% messageloop_start_time
-1.5% first_contentful_paint_time
-2.1% navigation_commit_time

A couple notes:
- setUsageAndCrashReportingFromNative was removed from UmaUtils because
  it is never used, and allowed for much more easily bringing UmaUtils
  into the base module.
- I wanted to make sure PureJavaExceptionHandler is still set up as
  early as possible, so left it in attachBaseContext. This required
  loading PureJavaExceptionReporter by reflection if an exception
  happens, since it has many hooks into chrome code.
- MainDexApplicationImpl is no longer needed since all the common code
  was moved to SplitCompatApplication

Bug: 1150600
Change-Id: Idd6c11293e47fa5e8bd6eb30c14535e60c204867
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570125
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833923}