Add an isCheckpointing() binder call for adb remount.

This is needed so "adb remount" can avoid writing to /data during a
checkpoint.

Bug: 157540389
Test: manual test
Change-Id: I33a691da3b99343acfc1e8ddf68a14504c3bfbe1
diff --git a/Checkpoint.cpp b/Checkpoint.cpp
index df5fc88..61035e5 100644
--- a/Checkpoint.cpp
+++ b/Checkpoint.cpp
@@ -294,6 +294,10 @@
     return false;
 }
 
+bool cp_isCheckpointing() {
+    return isCheckpointing;
+}
+
 namespace {
 const std::string kSleepTimeProp = "ro.sys.cp_msleeptime";
 const uint32_t msleeptime_default = 1000;  // 1 s