Don't append _asan to non-asan build flavors.

Right now we add the '_asan' suffix to the build flavor whenever
SANITIZE_TARGET is defined. This patch ensures the suffix is only
added when SANITIZE_TARGET=address.

Test: Checked flavor of SANITIZE_TARGET=address and integer_overflow
builds.
Bug: 30969751
Bug: 64561273

Change-Id: Ie20340f81b395e0b6b5f8086350bbc20f83d5fa4