[Android] envsetup: Simplify xwalk_android_gyp() a little.

Make it more in line with its android_gyp() counterpart by removing the
line saying which generator is being used (we always use ninja anyway),
and stop running gyp_xwalk in a subshell.
Merge pull request #1888 from rakuco/envsetup-simpler-xwalk_android_gyp

[Android] envsetup: Simplify xwalk_android_gyp() a little.