Make: Don't rely on presence of tools.jar in toolchain.

Some cts, doclava and javassist code uses private APIs that require
tools.jar to be on the classpath when building with the standard
OpenJDK 8 toolchain. However, OpenJDK 9 toolchains do not have this
file nor require it to be on the classpath. The reliance on the
presence of tools.jar means that AOSP can currently not build on
such toolchains.

After this CL, when EXPERIMENTAL_USE_OPENJDK9 is set, the value of
HOST_JDK_TOOLS_JAR is empty.

That flag can be set via:

export EXPERIMENTAL_USE_OPENJDK9=true

and unset via:

unset EXPERIMENTAL_USE_OPENJDK9

Test: make checkbuild (using OpenJDK 8 toolchain)
Test: manually confirmed that device boots
Bug: 38177295
Change-Id: Icc5451b51e44a574fd21024d434ed1fa8711de17