Apply ALLOW_MISSING_DEPENDENCIES to droiddoc template dir

Some of our minimal branches don't have the necessary droiddoc
templates, so don't cause a FindEmulator warning when the template dir
is missing and ALLOW_MISSING_DEPENDENCIES is set. The warnings will soon
be errors.

When the template directory is missing, ensure that the command doesn't
succeed by adding the directory into the dependency list. Passing a
missing directory doesn't cause an error otherwise, it just wouldn't use
the template.

Test: build-aosp_arm64.ninja is identical before and after this change
Test: Move away build/tools/droiddoc/templates-sdk; m -j libcore-docs
Change-Id: I122f3916b6ab348a5a98dcf2520992eda3655e26