Also fix a bug where the trace command may capture the wrong file
if multiple trace file paths are in the Android log buffer.
Previously we found a lower bound timestamp for the trace path log
by running the date command on the device and parsing the result on
the host. This could yield an inaccurate result if the device and
host are using different time zones.
The command will now obtain the most recent timestamp in the device's
time format by running logcat.
1) Moved basic utility code into base/ directory to make it clear which code
doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
aren't commands themselves.
This patch makes `flutter start` work without a clone of the engine git
repository. Making this work pulled a relatively large refactor of how the
commands interact with application packages and devices. Now commands that want
to interact with application packages or devices inherit from a common base
class that holds stores of those objects as members.
In production, the commands download and connect to devices based on the build
configuration stored on the FlutterCommandRunner. In testing, these fields are
used to mock out the real application package and devices.
