binder: freeze multiple contexts

binder freeze stops at the first context found for any pid, but
multiple ones are possible with the result that a process might end
up with inconsistent context states after freezing or unfreezing its
binder.

Freeze or unfreeze all contexts in a process upon a BINDER_FREEZE
ioctl.

Bug: 176996063
Test: verified that all contexts in a specific process with multiple
binders are frozen or unfrozen.
Signed-off-by: Marco Ballesio <balejs@google.com>

Change-Id: If0822e078e830e9fde10cc17b99e39ec7cf358d5