Add a phony target containing benchmarks for PGO

Bug: http://b/69281673
Bug: http://b/63768402

Add a 'pgo-benchmarks-hwui' target that builds hwuimicro and hwuimacro.
This lets us set an automated build/image that includes an instrumented
hwuimicro and hwuimacro for collecting profiles.

Test: Run 'make PRODUCT-taimen-userdebug ANDROID_PGO_INSTRUMENT=hwui
              pgo-targets-hwui'
      and verify that an instrumented hwuimacro and hwuimicro gets
      built.

Change-Id: I51308c1971e96f5c798b3303972b2f5e09103321