Disable from-text stub build for coverage build

Coverage builds depend on `native` properties for API elements, which
are not included in the API signature files and consequently in
from-text stubs. As no robust solution for handling this has been
planned out at the moment, from-text stub build is disabled for
coverage builds.

Per go/android-code-coverage-quickstart , Java code coverage is
enabled by the three environment variables: `EMMA_INSTRUMENT`,
`EMMA_INSTRUMENT_STAIC` and `EMMA_INSTRUMENT_FRAMEWORK`. This change
disables from-text stub build if any of the three variables are set
to true.

Test: go test ./java && m EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true nothing --build-from-text-stub and inspect ninja query to verify that the stub java library module depends on the from-source stub module
Bug: 304271961
Change-Id: Ie485c784145de6c253611e698354c4f9e4a30685