roll_aosp.sh can now generate an Android.bp file without having to
perform a roll. This facilitates on demand generation of Android.bp.
Provide an option to enable API traces with "--enableApiTrace" flag.

Usage -
    # Generates Android.bp and perform a roll
    ./scripts/roll_aosp.sh

    # Generates Android.bp
    ./scripts/roll_aosp.sh --genAndroidBp

    # Generates Android.bp with API tracing enabled
    ./scripts/roll_aosp.sh --genAndroidBp --enableApiTrace

Bug: angleproject:5774
Change-Id: I3a2bd505f5d75d6d5fc331d61fa9517d6042db70
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2774758Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>