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
Merged-In: I33a691da3b99343acfc1e8ddf68a14504c3bfbe1
5 files changed