ANDROID: KVM: arm64: Add protected_shared_mem statistic

When using nVHE in protected mode, protected memory can be shared
between host and a guest. Tracking this value is interesting from a
debug perspective to identify potential leaks.

Keeping the count of memory sharing is easy, each share/unshare will
return to the host where the accounting will take place.

Bug: 357781595
Bug: 278749606
Bug: 222044477
Change-Id: I43dcd258789f79dbfe489e5bf721e606c5e6e022
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>