ANDROID: Add flag to indicate compiling against ACK

Add a flag: __ANDROID_COMMON_KERNEL__ which out-of-tree vendor drivers
can use to check if they are compiling against an Android Common Kernel.
These out-of-tree vendor drivers can use this flag +
LINUX_KERNEL_VERSION to determine if a feature has been backported.

Bug: 229953929
Change-Id: I832344d63f3639479784753edfb7ac405068312f
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>