Add an aapt option to allow string variations for different devices.

The --product option to aapt is a comma-separated list of characteristics
of the device being built for.  For example, --product nosdcard,grayscale
for a device with no SD card and a grayscale screen.

Strings can specify a product="characteristic" option to cause that version
of the string to be used only for that type of device.  All such strings
should also specify, at the end of the block, product="default", which
will be used if none of the variations match.  For example:

<string name="choose" product="bw">Choose black or white</string>
<string name="choose" product="grayscale">Choose a shade of gray</string>
<string name="choose" product="default">Choose a color</string>

The default characteristic will also be used when no --product option
is specified.

Change-Id: Ie6c1505599e02e15b7818e8be6ec47bc6ce71aaa