Make ext build against core.platform.api.stubs

The ext target needs to be switched from core_current to depending on
core.platform.api.stubs (the default when no sdk_version is specified
and no_frameworks_libs = true) as it statically includes
libphonenumber-platform which itself needs to depend on
core.platform.api.stubs as it needs access to the
dalvik.annotation.compat.UnsupportedAppUsage annotation.

Without this change modifying the libphonenumber-platform target to
depend on core.platform.api.stubs causes a build failure.

Tested by changing libphonenumber-platform target to depend on
core.platform.api.stubs and running make checkbuild.

Bug: 117818301
Test: see above
Change-Id: I2b9154d22b67aafb57493b41b527818c37212c34