[Android] Return the correct context for external extensions

The previous implementation returns the context of runtime side which is
a context for library package. This is true for internal extensions. However,
for external extensions, the context should be the same as the activity
because they're inside of runtime client. This is to fix the issue to return
the activity as the context for external extensions.

BUG=#777
Merge pull request #954 from yongsheng/errorcontext

[Android] Return the correct context for external extensions