adb_gdb: Minor improvements

adb_gdb: Minor improvements

This patch improves the Android-specific adb_gdb debugging scripts
in several minor ways:

- It ensures that the temporary directory is always removed when
  the script exits, even whe it is interrupted with Ctrl-C.

- It ensures that if --pull-libs is interrupted by a Ctrl-C,
  the build.prop in /tmp/$USER-adb-gdb-libs/ is erased, forcing
  a --pull-libs operation in the next invokation, instead of keeping
  a portion of the libraries in a bad state.

- It adds a --su-prefix=<prefix> option, to be able to use tools
  like 'su' on rooted production devices. This is useful when the
  installed app is not debuggable (e.g. a release build), and there
  is no way to run ADB as root.

BUG=NONE

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=206990

[digit1, 2013-06-18 08:27:10]

Second patch handles bad --su-prefix values like "echo" that do work as a
command prefix, but don't actually give you root rights. Thanks qsr.

[digit1, 2013-06-18 08:28:02]

For context, these changes came out as useful during Benjamin's debugging of
Samsumg S4 - specific issues. The system image didn't allow to run adb as root,
but has a 'su' binary installed.

[qsr, 2013-06-18 08:31:55]

lgtm

[bulach, 2013-06-18 08:53:19]

lgtm

[Philippe, 2013-06-18 09:52:08]

lgtm, thanks David!

[commit-bot: I haz the power, 2013-06-18 10:37:16]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/digit@chromium.org/17103007/3

[commit-bot: I haz the power, 2013-06-18 14:24:47]

Change committed as 206990