ndk: Hard-code the TARGET_RELEASE

Trunk stable builds require a TARGET_RELEASE to be set, but this
variable doesn't have a real meaning for NDK builds.  So we
hard-core it here so individual build setups don't need to
configure it.

Bug: 309900087
Test: Treehugger
Change-Id: I17a2e463db0b561aea57417bc2b469070ba2062d