This CL gets capture/replay working on Android again.
* Updates where Android frame captures are written
* Uses debug system properties to prime Android environment variables
* Adds a configurable target Context to the capture_replay sample
* Updates capture/replay documentation for Android

Bug: angleproject:4036
Test: Captured TRex on Android, replayed on Linux
Change-Id: I94b4f6dc77468cd179b9d884b4dcd4afa56bd28c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1928056Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>