I was wondering since the Linux kernel in Android is under a free license while the other software is under Apache license. In which license we can say Android is?
Android is using multiple licenses and the overall collection is primarily under the Apache license but not only. Some parts are GPL, some BSD and some Apache (and many other FOSS licenses). The Linux Kernel itself is using the GPL for kernel space and practically any license you please when running a program in user space on top of the kernel.
Also, according to the GNU project website, Google should have made a copyright infringement by combining free software with code under Apache license. why this hasn't raise any illegal issues between Google and GNU the same way it did between Google and Oracle?
The fact that you redistribute a GPL-licensed kernel and a mostly Apache-licensed user space is fine and there is not a conflict there. This is the normal usage of the kernel. Several other user space tools are independent and therefore there is no intertwined code nor licensing in most cases.
The confusion raises from the fact that the Linux kernel in Android is under GPL v2.0 which is not compatible with Apache
This may be correct in the general case (yet always depends on how GPL and Apache-licensed interact and whether or not they end up being "separate programs"), but again -- this is something that has been explained and debated at great lengths by Linus and the community-- This is perfectly OK to run code on Linux in user space under any license, including proprietary ones. The kernel space copyleft does not bleed in user space in 99.999% of the cases. There are a few rare cases where it would bleed (e.g. if you would access kernel through a "non-standard" interface) but this is not a case here.