bp2build remove "-std" from cflags

Soong overrides "-std" flags when provided via cppflags or conlyflags;
however, any user-provided "-std=" cflag will be overridden by Soong's
std flag handling.

For Bazel, we _always_ allow user to override via user provided flags.
To prevent conflicts, we remove the silently ignored values from
Android.bp files in the bp2build conversion.

Test: build/bazel/ci/bp2build.sh
Test: build/bazel/ci/mixed_droid.sh
Change-Id: I4c33b2ae593a7ff3ff8e3ad15ef3461354fc0c83