[Disco][SRU][CVE-2019-2181] binder: check for overflow when alloc for security context

[Connor Kuehl]

https://people.canonical.com/~ubuntu-security/cve/2019/CVE-2019-2181.html

>From the above link:

	"In binder_transaction of binder.c in the Android kernel, there is a
	possible out of bounds write due to an integer overflow. This could lead to
	local escalation of privilege with no additional execution privileges
	needed. User interaction is needed for exploitation."

Clean cherry pick.