Change avd.py script to let developers point to emulator SDK.

Change avd.py script to let developers point to emulator SDK.

The emulator sdk root is now downloaded into the source tree (and .gitingore'd).
That means that doing a clobber on the src directory will delete the emulator
SDK downloaded by build/android/install_emulator_deps.py.

This change allows developers to have an emulator SDK located somewhere else
that they can point the avd.py script to. Also, some bots will remove the src
directory between builds. Rather than make the bots re-download the emulator sdk
everytime, this allows them to have an emulator SDK installed somewhere else and
point to it.

BUG=304129
TBR=craigdh@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=244637

[navabi, 2014-01-11 00:04:47]

This will allow developers and bots to do things like this:

ANDROID_EMULATOR_SDK_ROOT=<path_to_local_emulator_sdk> python
build/android/avd.py --abi=arm

to launch an emulator with their own emulator sdk.

[Maria, 2014-01-11 00:08:35]

lgtm

[commit-bot: I haz the power, 2014-01-11 00:22:02]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a lowly provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[kerz_chromium, 2014-01-13 19:45:02]

lgtm

[commit-bot: I haz the power, 2014-01-13 19:48:45]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/navabi@google.com/132793003/1

[commit-bot: I haz the power, 2014-01-13 23:39:48]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/navabi@google.com/132793003/1

[commit-bot: I haz the power, 2014-01-14 02:27:03]

Change committed as 244637