Remove Java PATH check, deprecate using PATH in Kati

The Java PATH fixup is now handled within soong_ui, along with the
values of ANDROID_JAVA_HOME based on OVERRIDE_ANDROID_JAVA_HOME /
EXPERIMENTAL_USE_OPENJDK9.

Mark PATH as deprecated, so that any reads/writes will cause warnings.
This will be switched to obsolete once it's verified that there are no
more users.

Using PATH within Kati means that we've got to rebuild the ninja files
whenever your PATH changes, which is not ideal, especially since some of
the envsetup functions can change your PATH. In most cases you only need
to use PATH within the bash portions of the build rules ($${PATH}), which
isn't treated as a make variable, so won't produce an error.

I'm also planning on replacing PATH in a future change with our own
directory that has placeholders for everything in your PATH. This will
let us remove tools that shouldn't be used from the build.

Test: m nothing
Test: build/soong/build_test.bash on AOSP and internal master
Change-Id: I18d8d19cfba313ff9176345bf73ac34e8dbebfbb