Pass TARGET_DEVICE_DIR from dumpvars to later kati runs

The find commands used to locate the BoardConfig.mk can't be optimized
by Kati, so we're currently spending ~125ms three times during every
build (dumpvars, cleanspec, and the main kati run). Preserve the value
of TARGET_DEVICE_DIR from the dumpvars run so that we only need to run
the find commands once.

Bug: 78020936
Test: out/build-taimen.ninja is identical
Test: out/soong.log shows that we're not running these finds again
Change-Id: Iee56b454c3661de2b58c161169218ecaf2135398