libdm: Make static std::string inline for DmTargetDefaultKey.

Bug: 149942628
Test: snapshotctl w/ ASAN build
Change-Id: I0f40b1f29e46bc9afaf19d8c21d62280e2c01074
Merged-In: I0f40b1f29e46bc9afaf19d8c21d62280e2c01074
2 files changed